tag:blogger.com,1999:blog-73788760347392562402014-10-01T22:26:47.116-07:00COMMON ELECTRICAL TERMS AND DEFINITIONSbosehttp://www.blogger.com/profile/14555558365540483390noreply@blogger.comBlogger1125tag:blogger.com,1999:blog-7378876034739256240.post-27682821188727196762009-05-21T08:31:00.000-07:002009-05-21T08:34:24.044-07:00Question and answers Electrical Maintenance Unit<br />- 1 -<br />1. What is static electricity?<br />Static electricity means electricity at rest. If we join a charged conductor to another<br />conductor, electricity flows from one to the other. This way an electric current is<br />produced, which lasts for a moment only. Static electricity is no use. Rubbing of two<br />different materials having different electrons produces this.<br />2. What is current?<br />Flow of electrons in any conductor is called electric current. Its symbol is ‘I’ and<br />measuring unit is Ampere measured by ammeter or ampere meter.<br />3. What is electro-motive force (emf) or voltage?<br />It is the pressure that moves the electrons to flow in any conductor. It is also known<br />as electromotive force voltage. Its symbol is ‘E’ or ‘V’ and measuring unit is volt<br />measured by voltmeter.<br />4. What is potential difference (P.D)?<br />The difference of potential between two points in a circuit is the voltage required to<br />drive the current between them or the voltage drop between those two points is<br />called the potential difference.<br />P.D = R * I volts.<br />5. What is terminal voltage (VT)?<br />It is the voltage available at the terminal of the source of supply. It’s symbol is VT.<br />VT = emf – P.D<br />6. What is resistance?<br />Resistance is the property of a substance, which gives opposition to flow of electrons<br />through itself. Its measuring unit is ohm and measured by ohmmeter, multi meter,<br />wheat stone bridge, and post office box. There are two types of resistances and they<br />are fixed resistance and variable resistance.<br />7. What is ampere?<br />The international ampere is defined as that steady current which, flowing through a<br />solution of silver nitrate, deposits silver at the rate of 0.001118 gm/sec.<br />8. What is volt?<br />The international volt is defined as 1/1.0183 of the emf of a Weston cadmium cell. It<br />is that difference of potential which, when applied to a conductor whose resistance is<br />1 (one) international ohm, will cause a current of 1 (one) international ampere to<br />flow.<br />Question and answers Electrical Maintenance Unit<br />- 2 -<br />9. What is matter?<br />The matter is defined as anything, which possesses weight and occupies space and<br />can be in any of three forms solid, liquid or gaseous. The matter consists of three<br />ingredients, which are protons, neutrons and electrons.<br />10. What is the speed of electricity or electrons?<br />The speed of electricity or electrons is 297842 km (186000 miles) per second.<br />11. How we get electric shock?<br />On all alternators, transformers neutral is earthed. Human body is conductor and<br />when touched to the live conductor it completes its shortest root though the body and<br />the body gets electric shock in which its nervous system, the heart, respiratory<br />system may cease to function.<br />12. What is fuse and what materials used for fuse wire?<br />Fuse is a weakest point in an electrical circuit, which melts when the excess current<br />flows through it in the electrical circuit.<br />The materials, which can be used in fuses, are tin, lead, zinc, silver, antimony,<br />copper, and aluminium, etc.<br />13. What is fusing factor?<br />The ratio of minimum fusing current and the current rating of fusing element is<br />called the fusing factor.<br />Fusing factor = minimum fusing current / current rating of fusing element. Its value<br />is always more than 1 (one).<br />14. What is soldering and what is brazing?<br />Soldering is the process of joining two metals with an alloy whose melting point is<br />less that of the materials to be soldered.<br />Soldering at high temperature using brass as solder is called brazing or hard<br />soldering.<br />The composition of the fine solder (soft solder) is tin 60% and lead 40%. Its melting<br />point is 190°C and is widely used.<br />15. What are the sources of electricity?<br />a. Battery (chemical source)<br />b. Generator (magnetism)<br />c. Thermocouple (heat generated)<br />d. Light (photo electric or solar cell)<br />e. Pressure (piezo electricity)<br />f. Friction (static electricity)<br />Question and answers Electrical Maintenance Unit<br />- 3 -<br />16. What are the effects of electric current?<br />a. Physical effect.<br />b. Chemical effect.<br />c. Magnetic effect.<br />d. Heating effect.<br />e. X-ray effect.<br />17. What is fire?<br />Destructive burning of any material is called the fire. Fire is the result of combining<br />fuel, oxygen and heat. If any one among three is separated the fire will come to end.<br />18. On what factor resistance of the substance depends (Laws of resistance)?<br />a. The resistance of the conductor is directly proportional to the length of the<br />conductor.<br />b. The resistance of the conductor is inversely proportional to the cross-section of<br />the conductor.<br />c. The resistance of the conductor depends on the nature of the material by which it<br />is made. That is specific resistance of the material.<br />d. The resistance of the conductor depends on its temperature.<br />The formula to find the resistance of the substance is below.<br />R = ρ L Ω<br />A<br />Where ρ is the constant for the material called its specific resistance or resistivity.<br />19. What is specific resistance or resistivity of the material?<br />Specific resistance of the material is the resistance of a piece of unit length and unit<br />cross-section (unit cube of that material). That is the resistance between the opposite<br />faces of unit cube of the material.<br />Or the specific resistance of any material is the resistance offered by the opposite<br />face of that material.<br />The unit of specific resistance is Ω/cm3, Ω/inch3, Ω/m3.<br />Question and answers Electrical Maintenance Unit<br />- 4 -<br />20. What is the temperature co-efficient of resistance?<br />Temperature co-efficient of the resistance of any substance is, change in its original<br />resistance due to the change in temperature. The temperature co-efficient of<br />resistance of material is the ratio of increase in resistance of 1°C rise in temperature<br />to the original resistance of the material (strictly at 0°C).<br />Formula for the resistance measurement is,<br />Rt = R0 (1+αt)<br />Where Rt 􀃆 Resistance at t°C.<br />R0 􀃆 Resistance at 0°C.<br />α 􀃆 Temperature co-efficient.<br />t 􀃆 Temperature rise.<br />21. What are the effects of temperature on resistance?<br />The effects of temperature on resistance are<br />a. In certain pure metals such as gold, copper, silver, aluminium etc. the resistance<br />increases with increasing temperature at fairly regular manner. Such metals<br />possess positive temperature co-efficient of resistance.<br />b. In certain materials (alloys) such as eureka, nichrome etc. the change in resistance<br />due to increasing temperature is irregular and negligible for a considerable range<br />of temperature.<br />c. In case of certain materials belongs to insulators, electrolytes such as paper,<br />rubber, glass, mica, carbon, acids, alkalies etc. the resistance decreases with<br />increasing temperature at fairly regular manner. Such materials posses negative<br />co-efficient of resistance.<br />22. What are the classifications of voltages?<br />a. Low voltage: Voltage not exceeding 250V. That is 0 – 250V.<br />b. Medium voltage: Voltage above 250V upto 650V comes under medium voltage.<br />c. High voltage: Voltage above 650V upto 33 kV comes under high voltage.<br />d. Extra high voltage: Above 33 kV voltages are extra high voltages.<br />23. What is coulomb?<br />It is the unit of charge. One (1) coulomb is the quantity of electricity, which is<br />circulated by a current of one (1) ampere in one second. The letter Q denotes it.<br />So that 1 coulomb = 1 amp * 1 second.<br />24. What is farad?<br />Farad is the unit of capacitance and the letter F denotes it. A condenser has a<br />capacitance of one (1) farad, if it is capable to maintain a charge of one coulomb<br />under a potential difference of one volt between its plates.<br />Question and answers Electrical Maintenance Unit<br />- 5 -<br />1 farad = 1 coulomb / 1 volt. = Q/V.<br />25. What is henry?<br />It is the unit of inductance and the letter H denotes it. A circuit has inductance of one<br />henry, if an electro-motive force of one volt if induced in that circuit, when the<br />current in that circuit changes at the rate of one ampere per second.<br />1 henry = 1 volt sec / ampere.<br />26. What is the least count of out-side micrometer?<br />The least count of out-side micrometer is 0.01mm.<br />Question and answers Electrical Maintenance Unit<br />- 6 -<br />27. State symbols for quantities and units.<br />Sl.No Name of the quantity Symbol Name of the unit Symbol<br />1 Volume V Cubic meter m3<br />2 Time T Second S<br />3 Frequency F Hertz Hz<br />4 Rotational frequency N Reciprocal second S-1<br />5 Slip S<br />6 Speed, Velocity V Meter per second m/s<br />7 Mass M Kilogramme Kg<br />8 Density P Kilogramme per cubic meter Kg / m3<br />9 Momentum P Kilogram meter per second Kg m/S<br />10 Force F Newton N<br />11 Weight G Newton N<br />12 Torque T Newton meter Nm<br />13 Pressure P Newton per square meter N/ m2<br />14 Work W Joule J<br />15 Energy E,W Joule J<br />16 Power P Watt W<br />17 Efficiency η<br />18 Electric charge Q Coulomb C<br />19 Emf, Voltage, PD E Volt V<br />20 Electric flux ψ Coulomb C<br />21 Capacitance C Farad F<br />22 Electric current I Ampere A<br />23 Magneto motive force Fm Ampere turns AT<br />24 Magnetic flux density B Telsa T<br />25 Magnetic flux ϕ Weber Wb<br />26 Self inductance L Henry H<br />27 Mutual inductance Lmm, m Henry H<br />28 Resistance R Ohm Ω<br />29 Resistivity ρ Ohm meter Ωm<br />30 Conductance G Mho<br />31 Reluctance S Reciprocal henry H-1<br />32 Impedance Z Ohm Ω<br />33 Reactance X Ohm Ω<br />34 Admittance Y Mho<br />35 Active power P Watt W<br />36 Reactive power Q VAR VAR<br />37 Apparent power S Volt-ampere VA<br />38 Number of turns N<br />39 Speed N Rotation per minute r.p.m<br />40 Number of phases M<br />41 Number of pair of poles P<br />42 Luminous intensity L Candela Ca<br />43 Luminous flux φ Lumen lm<br />44 Quantity of light Q Lumen second lm S<br />Question and answers Electrical Maintenance Unit<br />- 7 -<br />45 Illumination E Lux lx<br />Question and answers Electrical Maintenance Unit<br />- 8 -<br />28. State Greek alphabets and what for they are used?<br />Sl.No Symbol Name Used for to indicate<br />1 α Alpha Angle, temperature co-efficient of resistance<br />2 β Beta Angle<br />3 γ Gamma Angle, conductivity<br />4 δ Delta<br />5 η eta Efficiency<br />6 θ Theta Angle, temperature<br />7 λ Lambada Wave length<br />8 μ Mu Amplification factor<br />9 π Pi 22/7<br />10 ρ Rho Specific resistance, resistivity<br />11 σ Sigma Charge density, fractional slip<br />12 φ Phi Phase angle<br />13 ϕ Capital phi Magnetic flux<br />14 Ψ Psi<br />15 ψ Capital psi Electric flux<br />16 ω Omega Angular velocity<br />29. What is conductance?<br />Conductance is the property of the conductor, which allows the flow of electric<br />current through it. Conductance is denoted by the letter G and is reciprocal of<br />resistance. The unit of conductance is mho. A substance, which posses conductance<br />as its major property can be called as a good conductor.<br />30. What you mean by insulator? What are the qualities of good insulator?<br />A substance, which will not allow the flow of electric current to pass through it is<br />called the insulator. The conductance and conductivity is zero in insulators.<br />Insulators are used to isolate the electric current from neighbouring parts. Insulators<br />will not allow the leakage of current, short-circuiting current, shock to the operator<br />and isolates the electric current safely with out any diversion to any other place.<br />Qualities of good insulator<br />a. It should be flexible<br />b. It should have good mechanical strength<br />c. It should easily moulded into any shape<br />d. It should not be effected by acid<br />e. It should be non-inflammable<br />f. It should have very high specific resistance to prevent leakage current<br />g. It should be withstand high temperature. Because insulators posses negative temperature coefficient<br />of resistance. That is resistance decreases with increasing temperature<br />h. It should have high dielectric strength<br />Question and answers Electrical Maintenance Unit<br />- 9 -<br />31. What is electrode?<br />A conducting element used for converging (centering) current to and from a medium<br />is called electrode. There are two types of electrode. A positive and other is negative.<br />Question and answers Electrical Maintenance Unit<br />- 10 -<br />32. What is force?<br />Force is that which charge or tends to change a body state of rest or uniform motion<br />through a straight line. The unit of force is Newton.<br />33. What is Newton?<br />One Newton is that amount of force which acting on one-kilogram mass for one<br />second gives an acceleration 1 meter/sec/sec.<br />34. What is weight?<br />Weight is the gravitation force by which a body attracted to the earth. Gravitational<br />unit of force in M.K.S system is kilogram weight or 9.81 Newton.<br />Weight is the force with which 1-kilogram mass is attracted by the earth towards its<br />center.<br />35. What is bayer?<br />Bayer is the C.G.S unit of pressure and is equal to 1-dyne/cm2.<br />36. What is conductor?<br />Substances such as metals, which have large number of free electrons are said to<br />offer a low resistance to the flow of electrons under the influence of emf and hence<br />are called conductors.<br />Conductors are used to conduct electricity from one place to another place due to its<br />major property conductance. Conductors are classified into three main groups.<br />a. Good conductors.<br />b. Semi conductors.<br />c. Fair conductors.<br />37. What are the properties of good conductor?<br />Properties of good conductor<br />a. It posses very low resistance or specific resistance.<br />b. It posses more conductance and there by conducts electricity readily through it.<br />c. It is a good conductor of heat.<br />d. It is highly resistance to corrosion by liquid.<br />e. It must be malleable and ductile.<br />f. It must be flexible.<br />g. It posses better tensile strength.<br />h. It should not react with climatic conditions.<br />i. It can be drawn in very fine wires.<br />j. It must be readily joinable.<br />k. It must be very low in cost.<br />l. It must available in plenty.<br />Question and answers Electrical Maintenance Unit<br />- 11 -<br />37. What are the common conductors in sequence with high conductivity?<br />a. Silver<br />b. Silver copper alloy<br />c. Copper (Hard down and Annealed)<br />d. Gold<br />e. Zinc<br />f. Platinum<br />g. Tin<br />h. Aluminum<br />i. Iron<br />j. Brass<br />k. Phosphorous bronze<br />l. Nickel<br />m. Lead<br />n. Germanium silver<br />o. Antimony<br />p. Platinoid<br />q. Mercury<br />r. Bismuth<br />s. Platinum iridium<br />38. What is semiconductor?<br />Semiconductors posses less conductivity (conductance) than good conductors. That<br />is semiconductors gives opposition (resistance) to the flow of free electrons than that<br />of good conductor.<br />Examples for semiconductor are<br />a. Dilute acid<br />b. Metallic ores<br />c. See water<br />d. Moist earth<br />e. Silicone<br />f. Germanium<br />39. What is fair conductor?<br />Fair conductors are the materials, which have less conductivity than that of<br />semiconductor. Fair conductor gives more opposition to the flow of free electrons<br />than that of semiconductors.<br />Examples for fair conductors are<br />a. Charcoal<br />b. Coke<br />c. Carbon<br />d. Plumbago<br />40. What is resistor?<br />Question and answers Electrical Maintenance Unit<br />- 12 -<br />Resistors posses high resistance, but less conductance. This property is well utilized<br />to convert electrical energy into heat energy. Common application of resistors is<br />production of heaters. Examples are eureka, carbon, nichrome, tungsten, manganin,<br />germanium, and tentalum. In case of heaters, electrical iron and soldering iron etc the<br />heating element are made of nichrome, but in lamps filament is made of tungsten.<br />Question and answers Electrical Maintenance Unit<br />- 13 -<br />41. What is the difference between resistor, rheostat and potential divider?<br />Resistor: a fixed resistance connected permanently in the circuit for limiting the<br />current to definite value is called the resistor.<br />Rheostat: a variable resistance by sliding contacts on it the current can be varied is<br />called rheostat.<br />Potential divider: when a resistance is used to develop a voltage drop it is called a<br />potential divider.<br />42. What is solder?<br />Solder is an alloy of lead and tin mixed in different proposition as per the work to be<br />done. In some cases certain % of bismuth and cadmium is also added to decrease the<br />melting point of the solder. Antimony increases the melting point of the solder.<br />Bismuth has a special quality in comparing to most of other metals. That is it<br />expands when it cools. This property helps to shrink the solder and there by it allows<br />the joint become firm.<br />The quality of the solder depends on the % of tin in the solder. To get stronger joint<br />add more tin in the solder.<br />For electrical work fine solder of 1½ part tin and 1 part lead is used and for sheet<br />metal works soft solder of 1 part tin and 1 part lead is used.<br />43. What is flux?<br />Flux is a cleanser and is used to remove and prevent oxidation of the metals,<br />allowing the solder to flow from and to, to unite the solder more firmly with the<br />surface to be joined.<br />44. What is skin effect?<br />Electricity has affinity (fondness) to pass through peripheral surface of the<br />conductor. This effect of electricity flowing through the peripheral surface of the<br />conductor is known as skin effect.<br />45. What are the advantages of stranded cables?<br />a. It gives flexibility.<br />b. It prevents skin effect.<br />c. Increases current carrying capacity.<br />d. It provides easy in soldering joint.<br />e. If one strand breaks the other will carry the load current.<br />Question and answers Electrical Maintenance Unit<br />- 14 -<br />46. State the Ohm’s law.<br />In a closed electrical circuit, at a constant temperature, the ratio between the resulting<br />unvarying current or direct current and applied voltage is a constant. That constant is<br />known as resistance.<br />OR<br />In simple manner Ohm’s law says that, in a closed electrical circuit the current is<br />directly proportional to the voltage and inversely proportional to the resistance of the<br />circuit.<br />I = V/R or<br />R = V/I or<br />E = IR.<br />Ohm's Law / Power Formulas<br />P = watts<br />I = amps<br />R = ohms<br />E = Volts<br />47. What is series circuit? What are the characteristics of series circuit?<br />It is that circuit where two or more electrical consuming devices are connected so as<br />to provide only one path to the flow of current.<br />Characteristics of series circuit<br />a. It has only one path for the flow of current.<br />b. If any breakage happens the whole system will be out of that circuit.<br />c. It is very difficult to find the fault.<br />d. Individual voltage drop depends on individual resistance (V = I rn).<br />e. The total resistance of a series circuit is the sum of the individual resistance.<br />f. Addition of the resistance increases total resistance and decreases the current.<br />g. Individual device will not get its full efficiency.<br />Question and answers Electrical Maintenance Unit<br />- 15 -<br />48. What is parallel circuit? What are the characteristics of parallel circuit?<br />It is that circuit where two or more electrical consuming devices are connected so as<br />to provide as many parallel paths to the flow of current.<br />Characteristics of parallel circuit<br />a. As many parallel paths as there are devices.<br />b. Individual devices will get its full efficiency.<br />c. Breakage in one circuit will not affect the other circuit.<br />d. Current in each device is different according to the resistance of the device.<br />e. If the individual resistance increases the total resistance will decrease<br />(1/R = 1/ r1 + 1/ r2 +1/ rn)<br />f. The reciprocal of total resistance is equal to the sum of the reciprocal of<br />individual parallel resistances (1/R = 1/ r1 + 1/ r2 +1/ rn).<br />g. Individual conductance is inversely proportional to the individual resistance.<br />h. If two same value resistors are connected in parallel circuit the total resistance is<br />the resistance of one resistor. And the total of parallel circuit resistance will be<br />less than the least resistance in that circuit.<br />49. What is capacitor? On what factor capacity of a capacitor depends?<br />Capacitor or condenser is a device to store electrical energy and to release it into the<br />circuit of which the capacitor forms a part.<br />Capacity of a capacitor depends on following factors<br />a. Capacity of the capacitor is directly proportional to the area of the plate.<br />b. Capacity is inversely proportional to the distance between the plate. That is if the<br />distance is more the capacity decreases or if the distance is less the capacity more.<br />c. It depends on the nature of dielectric constant.<br />50. On what factor voltage rating of the capacitor depend?<br />The voltage rating of the capacitor depends on the distance between the plates of the<br />capacitor. If the voltage exceeds, the electrons across the space between the plates<br />can result in permanent damage to the capacitor.<br />51. What are the types of capacitor?<br />a. Paper capacitor.<br />b. Rolled plastic cover or polyester type capacitor.<br />c. Mica capacitor.<br />d. Silver mica capacitor.<br />e. Ceramic capacitor.<br />f. Electrolytic capacitor.<br />52. What is the resultant capacitance in series and parallel circuit?<br />In series circuit the resultant capacitance 1/CT = 1/c1+1/c2 + 1cn farad.<br />In parallel circuit the resultant capacitance CT = c1 + c2 + cn farad.<br />Question and answers Electrical Maintenance Unit<br />- 16 -<br />53. What is the formula to find the capacitance in a circuit?<br />C = Q/E farad.<br />Question and answers Electrical Maintenance Unit<br />- 17 -<br />54. What is work?<br />Work is said to be done, when the point of application of the force moves. Work<br />done is equal to force * distance. The unit of work is Newton (M.K.S system) and<br />joule (1 Newton Meter).<br />55. What is power?<br />Power is the rate of doing work or power is the work done per second.<br />Power = Work / time.<br />Unit of electrical power is watt. One mechanical horsepower is equal to 746 watts<br />(British) and 735.5 watts (metric) or 735.5 joules/sec. So 1 kW is equal to 1.34<br />horsepower (British) and 1.36 horsepower (metric).<br />56. What is energy?<br />Energy is the capacity to do the work. The unit of energy is joule or watt-second or<br />watt-hour or kilo watt-hour.<br />57. Define Joule’s law.<br />The heat generated in conductor (resistance) while the flow of current is directly<br />proportional to the square of the current, the resistance of the conductor and time for<br />which the current flows.<br />H = I2 R t/J calories.<br />Where J is mechanical equivalent of heat is equal to 4.2 Joules.<br />In electricity H = 0.24 I2 R t calories.<br />58. What is electrolysis?<br />When current passes through an acid or a salt, it de-composes and the two decomposed<br />portions tend to move in opposite direction. This process is called the<br />electrolysis.<br />Or the process of decomposing a liquid by the passage of electric current (DC)<br />through it is called the electrolysis or electric analysis.<br />59. What are the Faradays laws of electrolysis?<br />First law<br />The mass ions liberated at an electrode are directly proportional to the quantity of<br />electricity (coulomb Q) which has passed through the electrolyte. That is M∝Q or<br />M∝I t.<br />And M = Z I t.<br />Where Z is electro chemical equivalent.<br />Second law<br />If the same quantity of electricity passes through several electrolyte the masses of the<br />ions liberated are proportional to their respective chemical equivalent.<br />Question and answers Electrical Maintenance Unit<br />- 18 -<br />60. What is electro plating?<br />The process of depositing a metal on the surface of another metal by electrolysis is<br />known as electro plating. Usually the plating material will be silver, chromium etc.<br />Question and answers Electrical Maintenance Unit<br />- 19 -<br />61. What are the applications of electrolysis?<br />a. Electro plating.<br />b. Purification of copper and extraction of number of metals and number of<br />commercial compounds like sodium, hydrogen, hydroxide, oxygen etc.<br />c. Electro typing.<br />d. Determination of DC polarity.<br />e. Electro refining of metals.<br />62. State the laws of magnetism.<br />a. Magnet imparts its magnetic properties to other metals.<br />b. When a magnet is suspended freely horizontally, it stands at geographical north<br />and south.<br />c. Every magnet has a north and its associated separable South Pole.<br />d. If a magnet broken in any number of pieces, each piece will act as a separate<br />magnet having north and south poles.<br />e. Like poles repulse and unlike poles attracts.<br />f. The amount of attraction or repulsion is directly proportional to the pole<br />strength and inversely proportional to the square of the distance between them.<br />This is some times known as inverse square law.<br />63. What is flux density?<br />It is the flux passing per unit area in a substance through a plain at a right angle to<br />the flux. The letter ‘B’ denotes it and it is measured in Weber/cm2.<br />B = Q/a Weber/cm2.<br />64. What is magneto motive force?<br />The force, which drives the magnetic flux through a magnetic circuit, is called the<br />magneto motive force.<br />65. What is permeability?<br />Permeability of a substance is the conducting power for lines of force of magnetic<br />material as compared with the air.<br />66. What is reluctivity?<br />It is the specific reluctance of a magnetic circuit or magnetic material as in the case<br />of resistivity in an electric circuit.<br />67. What is reluctance?<br />It is the property of a magnetic material, which opposes the establishment of<br />magnetic flux in it, as in the case resistance in an electric circuit.<br />68. What is permeance?<br />Question and answers Electrical Maintenance Unit<br />- 20 -<br />It is the reciprocal of reluctance, which helps to develop or establish magnetic flux<br />easily in a magnetic material as in the case of conductivity in an electrical circuit.<br />Question and answers Electrical Maintenance Unit<br />- 21 -<br />69. What are the methods of magnetization?<br />a. Tough method<br />b. By means of electric current<br />c. Induction method<br />70. How the polarity of the magnet can be determined?<br />Polarity of the magnet can be determined by ‘End rule’ and ‘Palm rule’.<br />71. What are the advantages of electro magnetism?<br />a. Electro magnets can be magnetised very easily by sending DC through it.<br />b. Changing the direction of the current through the coil can change the polarity of<br />the poles.<br />c. The strength of the magnet can be controlled by the electric current.<br />d. Electro magnets can be made in any shape depending upon the need.<br />e. The magnetic strength remains constant as long as the current is constant.<br />72. State ‘Cork screw rule’ and ‘Right hand thumb rule’.<br />Cork screw rule<br />Direction of magnetic lines of force around a straight current carrying conductor can<br />be determined by these rules.<br />‘Cork screw rule’ says that, the direction of magnetic lines of force around a straight<br />current carrying conductor is the same as that in which the cork screw must be<br />rotated to cause to an advance in the direction of the current in conductor.<br />Right hand thumb rule<br />Grasp the conductor with right hand in such a way that the extended thumb must be<br />in the direction of current in the conductor. Then the folded fingers or encircling<br />fingers must be in the direction of magnetic lines of force around the conductor.<br />73. Who discovered electro magnetism?<br />‘Orsted’ a denish scientist discovered that whenever an electric current passes<br />through a conductor, a magnetic field will be produced around that conductor in<br />concentric circle. In addition to that heat will be produced in that conductor.<br />74. State the faraday’s laws of electro magnetic induction.<br />In 1831 Faraday discovered the production of electric current in electric conductor<br />by converting magnetism. Faraday has mentioned two laws known as faraday’s laws<br />of electro magnetic induction.<br />First law<br />Whenever a conductor causes to cut the magnetic lines of force an emf will be<br />induced in that conductor.<br />Second law<br />Question and answers Electrical Maintenance Unit<br />- 22 -<br />The quantity of electricity or the value of the emf produced in that conductor is<br />directly proportional to the rate of change of flux linked with that conductor.<br />Question and answers Electrical Maintenance Unit<br />- 23 -<br />75. How we can find the direction of induced emf?<br />The direction of induced emf can be find out by the ‘Fleming’s right hand rule’, and<br />‘lenz’s law’<br />Fleming’s right hand rule<br />Fleming’s right hand rule states that, if one extends the thumb, fore finger and<br />middle finger of the right hand at right angle to each other in such a way that the<br />thumb point in direction of motion of the conductor, the fore finger in the direction<br />of flux (from north to south pole), then the middle finger is indicate the direction of<br />the induced emf in the conductor.<br />Lenz’s law<br />The lenz’s law states that, electro magnetically induced current always flows in such<br />a way or direction that the action of magnetic field set up by induced current tends to<br />opposes the root cause which produces it.<br />76. What is eddy current?<br />Eddy currents are those which are produced or induced in the mass of metal<br />whenever the metal are moved in magnetic field of the magnetic field is moved<br />across the mass metal so as to link it. The direction of this eddy current is always in<br />opposite direction to the cause to produce them as per lenz’s law.<br />Eddy current can be calculated by following equation<br />We = k Bmax<br />2 f2 t2 v watt.<br />Where k – Constant<br />Bmax – Maximum flux density<br />f – frequency of magnetic reversal<br />t – thickness of each lamination<br />v – volume if the armature core or mass metal.<br />Development of eddy current is made use in energy meters to provide controlling<br />torque and also in form of automatic starters in moving coil measuring instruments.<br />77. What is magnetic Hysteresis?<br />Lagging of magnetization or induction flux density ‘B’ behind the magnetising force<br />‘H’ is known as magnetic hysteresis.<br />78. What are the types of induced electro motive force?<br />a. Dynamically induced emf.<br />b. Statically induced emf.<br />Statically induced emf can be further divided into two groups.<br />a. Mutually induced emf.<br />b. Self induced emf.<br />Question and answers Electrical Maintenance Unit<br />- 24 -<br />79. What are the use of mutual induction and self-induction?<br />Use of mutual induction<br />a. Transformers are works on this principle.<br />b. An inductance furnace makes use of it.<br />c. Used in ignition coils of motor car, motor cycles, scooters etc.<br />Use of self-induction<br />a. In regulators to give reduced voltage to the fans.<br />b. In fluorescence tube light to give high voltage at the time of starting and to give<br />law voltage at it’s normal working time.<br />c. Used in welding plant rectifiers to keep arc stationary by smoothing choke.<br />80. What are the different methods used to measure the resistance?<br />The different methods developed to measure the resistances are as follows.<br />a. Wheat stone bridge.<br />b. Slide wire bridge.<br />c. Post office box.<br />d. Ohm meter.<br />e. AVO meter or multi meter.<br />f. Bridge megger.<br />g. Megger.<br />81. What is generator? What are the essential parts of the generator?<br />Generator is a machine, which converts mechanical energy into electrical energy.<br />A generator works on under the principle of faraday’s laws of electro magnetic<br />induction.<br />It’s essential parts are conductor, magnetic field and the movement of either the<br />conductor or the magnetic field so as to create a rate of change of flux linkage with<br />the conductor by the action of applied mechanical energy.<br />82. What is the equation used to find out frequency of number of cycles of induced emf?<br />f = NP/120<br />83. What are the types of generators?<br />There are two types of generator.<br />a. Permanent magnet generator.<br />b. Electro magnet generator.<br />In electro magnet generator there are two types.<br />a. Self excited generator.<br />b. Separately excited generator.<br />Question and answers Electrical Maintenance Unit<br />- 25 -<br />84. What are main types of DC generator?<br />Mainly there are three types.<br />a. Series generator or series wound generator.<br />b. Shunt generator or shunt wound generator.<br />c. Compound generator.<br />There are different types of compound generator.<br />a. Short shunt commulative compound generator.<br />b. Short shunt differential compound generator.<br />c. Long shunt commulative compound generator.<br />d. Long shunt differential compound generator.<br />Depending upon the terminal voltage characteristics there are three types of<br />compound generator.<br />a. Under compound generator.<br />b. Flat or level compound generator.<br />c. Over compound generator.<br />85. What is the emf equation for generator?<br />emf = P * φ * Z * N / A * 60<br />Where,<br />φ = Flux per pole in Weber.<br />Z = Total number of armature conductors.<br />P = Number of poles.<br />A = Number of parallel paths in armature.<br />N = Speed in rpm.<br />emf = emf generated in one parallel path and it is the emf generated of that generator.<br />For a wave wound generator there are only two (2) parallel paths in the armature. In<br />such cases A=2 and in lap wave wound armature parallel paths is equal to the<br />number of poles in the armature winding.<br />86. What are the losses in DC generator?<br />There are two main losses.<br />a. Copper losses or electrical losses.<br />b. Stray losses or rotational losses or constant losses.<br />Copper losses includes following losses<br />a. Armature copper losses (Ia<br />2 ra).<br />b. Field copper losses (Ise<br />2 rse) or (Ish<br />2 rsh).<br />c. Losses in brush.<br />Stray losses are as follows<br />a. Magnetic losses (Iron loss or core loss).<br />b. Mechanical losses.<br />Question and answers Electrical Maintenance Unit<br />- 26 -<br />87. What is efficiency of generator?<br />Efficiency = Out put / input<br />= Out put / out put + losses<br />Question and answers Electrical Maintenance Unit<br />- 27 -<br />88. What is armature reaction?<br />Armature reaction is the effect of armature flux on the main field flux.<br />The effects of armature reaction are follows.<br />a. Armature reaction destroys (cross magnetizes) and weakens the main field flux<br />produced by the main pole.<br />b. It causes to reduce the induced emf in the armature.<br />c. It causes to reduce the efficiency of machine.<br />d. It causes to produce sparking at the brushes due to the shifting of M.N.A<br />(magnetic neutral axis).<br />e. At short-circuited loads or at very heavy loads, in case of self-excited generators<br />de-magnetising of pole cores (wiping of residual magnetism) may takes place.<br />89. What are the remedies for armature reaction?<br />a. Brushes have to shift to new M.N.A position in the direction of rotation of<br />armature.<br />b. To over come the weakening of the field extra turns have to be added in armature.<br />c. Pole shoes have to modify at trailing pole tip side to increase the reluctance.<br />d. Pole shoes have to modify to increase the reluctance.<br />e. In big machines there is chance of load fluctuation, a compensating winding to be<br />placed at the pole shoes and it is connected in series with the armature winding<br />such that the current in that winding is opposite to the armature winding.<br />90. What is commutation?<br />Usually the width of the brush is equal to the two segments of the commutator.<br />Whenever a brush contacts two or more commutator segments, the connected to<br />those segments are short-circuited. After the period of short-circuiting the current on<br />those coils changes their current direction in it. The change that takes place in the<br />coil after the period of short-circuiting of that coil is called commutation.<br />When that changes take place slowly, that commutation is known as smooth<br />commutation and when that changes take place suddenly, that commutation is known<br />as rough commutation.<br />If the commutation is not smooth, the spark may be more and that will damage the<br />commutator surface, commutator segments and so the winding.<br />The remedies for rough commutation are resistance commutation method and emf<br />commutation method.<br />91. What are the characteristics of DC generator?<br />There are three main characteristics of DC generator and they are,<br />a. No load saturation characteristics or OCC or magnetic characteristics (E0/If).<br />b. Internal or total characteristics (E/Ia).<br />c. External characteristics (V/I).<br />Question and answers Electrical Maintenance Unit<br />- 28 -<br />92. What is motor? How DC motor works?<br />A motor is a machine, which takes electrical energy and converts that electrical<br />energy into mechanical energy.<br />DC motor works under the principle, that whenever a current carrying conductor<br />placed in a magnetic field, a mechanical force will be acts upon that conductor and<br />the conductor tends to rotate, if it is arranged freely to rotate.<br />The direction of the force or rotation can be determine by “Fleming’s left hand rule”<br />93. What is torque?<br />Whenever a current carrying conductor placed in a magnetic field, a mechanical<br />force will be acts upon that conductor and the conductor tends to rotate, if it is<br />arranged freely to rotate. This rotation is due to the turning or twisting force acted on<br />that conductor. This turning or twisting movement of a force about an axis is called<br />torque ‘T’.<br />T = force * radius Newton-meter.<br />Work done per revolution = force * distance covered in one revolution.<br />∴ Work done per revolution = force * 2πr.<br />Work done per second = force * 2πr N (r.p.s)<br />Work done per second = 2π N T (äT = F * r)<br />So power developed in metric horsepower is equal to force 2πNT/735.5 hp.<br />94. What are the classifications of DC motor?<br />a. DC series motor.<br />b. DC shunt motor.<br />c. DC compound motor.<br />There are two types of DC compound motor.<br />a. Differential compound motor.<br />b. Commulative compound motor.<br />95. What are the losses in DC motor?<br />The losses in DC motor are same as that of DC generator. They are copper losses,<br />magnetic losses and mechanical losses.<br />96. What are the characteristics of DC motor?<br />The characteristics of DC motor shows the relation between armature current (Ia),<br />speed (N) and torque (T).<br />a. Torque and armature current characteristics. It is also known as electrical<br />characteristics.<br />b. Speed and armature current characteristics.<br />c. Speed and torque characteristics.<br />Question and answers Electrical Maintenance Unit<br />- 29 -<br />97. What is the necessity of DC motor starter?<br />Eb = V – Ia ra.<br />∴ Ia = V – Eb / ra.<br />At the time of starting from the rest there is no any back emf (Eb) in the armature. So<br />a large current flows through the armature based on V / ra. This very large current<br />blow out the fuses and before to that it will damage the commutator, commutator<br />brushes and winding. To avoid this difficulties a proper resistance has to be<br />introduce in series with the armature till the motor reaches it’s rated speed or till<br />development of Eb in the armature to reduce the starting large current to safe value.<br />This starting resistance is gradually cut out as the motor gains speed and the<br />develops back emf (Eb) which regulates it’s speed and armature current. This can be<br />achieved by the help of starter.<br />98. What are the types of DC motor starter?<br />a. DC two point starter for series motor.<br />b. DC three point starter for shunt motor.<br />c. DC four point starter for compound motor.<br />99. How speed control of DC motor can be achieved?<br />Induced emf in the armature E = P * φ * Z * N / A * 60 volts.<br />Where Z and A are constant.<br />N ∝ Eb / φ<br />N ∝ V – Ia ra / φ.<br />We can consider that the Ia ra drop is very small and there by in the place of V – Ia ra<br />we can consider only V. If it so then N ∝ V/ φ.<br />So speed may be varied by varying either applied voltage to the armature and by<br />varying field flux or field strength per pole or total field flux.<br />100. What is cell?<br />Cell is one unit for converting chemical energy into electrical energy. A cell<br />essentially requires two electrodes, electrolyte and container.<br />101. What is battery?<br />The combination of two or more cells is called the battery.<br />102. What are the classifications of cell?<br />a. Primary cells.<br />b. Secondary cells.<br />Question and answers Electrical Maintenance Unit<br />- 30 -<br />103. What are the differences between primary cell and secondary cell?<br />Primary cells are those cells, which cannot be re-charged after the substances<br />(electrolyte, electrode and container) used in it becomes useless.<br />The common primary cells in use are,<br />a. Simple voltaic cell (one fluid cell).<br />b. Daniel cell (two fluid cell).<br />c. Leclanche cell (two fluid cell).<br />d. Dry cell.<br />e. Standard cell or Weston cadmium cell.<br />Secondary cells are those cells, which can be re-charged and use again once they<br />discharged or used for the work for number of times with out re-newing it’s<br />materials.<br />Most commonly used secondary cells are,<br />a. Lead acid cell.<br />b. Nickel iron alkaline cell.<br />c. Nickel cadmium alkaline cell.<br />104. What is polarization? What is local action?<br />Polarization<br />The hydrogen bubbles which are clinging over the surface of copper electrode<br />(anode) becomes a thin film of hydrogen over the copper electrode. This hydrogen<br />film increases the internal resistance and reduces the emf of the cell and hence the<br />cell soon becomes inactive. This effect is known as polarization.<br />Local action<br />In voltaic cell it is observed that, when the cell is not connected to the load and not<br />supplying any current zinc will continuously dissolving in the electrolyte. This is<br />due to the impurities (copper, iron, tin, and lead) in the commercial zinc. So that<br />whenever commercial zinc is used as a electrode, separate small cells are<br />developed between the impurities and zinc with the presence of electrolyte. These<br />local cells consume always zinc and the emf developed by those local cells are<br />always opposite to the main emf. The action of these cells is known as local action.<br />105. What are the advantages of secondary cells over primary cell?<br />a. It gives high current capacity.<br />b. Its internal resistance is very low.<br />c. It gives a constant current.<br />d. It posses very high efficiency.<br />e. It posses fairly constant emf.<br />f. It posses good mechanical strength.<br />g. It posses large storage capacity.<br />Question and answers Electrical Maintenance Unit<br />- 31 -<br />h. It can be renewed by charging after it is discharge.<br />i. It is durable.<br />Question and answers Electrical Maintenance Unit<br />- 32 -<br />106. What is Plante plate and Faure plate?<br />There are two types of positive plate preparation. They are Plante plate and Faure<br />plate.<br />Plante plate<br />As per plante process positive plate PbO2 are prepared by a process of repeated<br />charging and discharging of pure lead. Positive plates, which are made by this<br />process, are also called ‘formal plates’. This process of positive plate preparation<br />required very long time for it’s manufacturing and so it is very costly.<br />Faure plate<br />Faure plates are generally made up of rectangular lead grid into which the active<br />material lead peroxide PbO2 is filled in the form of paste.<br />107. How negative plate is made up of?<br />The negative plate of a lead acid cell is made up of spongy lead ‘Pb’. The negative<br />plates are also of rectangular lead grid and the active material Pb in the form of<br />paste is held firmly in this lead grid.<br />108. Why negative plates are one more than positive plates?<br />Negative plates are one more than positive plates so as to get negative plates on<br />both the sides of positive plates. This is to prevent the buckling action of the lead<br />on positive plate in the multi plate lead acid cell. The other reason is that both the<br />sides of positive plates will become active and the efficiency of the positive plate<br />and the cell will increase.<br />109. What is electrolyte?<br />Electrolyte is the medium through which the current produces chemical changes.<br />Electrolyte is a mixture of sulphuric acid o 1.85 specific gravity (concentrated<br />sulphuric acid) diluted with distilled water in the ratio of 1:3 approximately, so the<br />specific gravity of the dilute sulphuric acid is 1.280.<br />110. What are the types of grouping of cells?<br />There are three main ways of grouping.<br />a. Series grouping.<br />b. Parallel grouping.<br />c. Series parallel grouping.<br />Question and answers Electrical Maintenance Unit<br />- 33 -<br />111. What are the advantages of series grouping and parallel grouping?<br />Advantages of series grouping.<br />a. The total emf increases and is equal to ‘nE’. Where n – total number of cells in<br />series and E – emf of one cell.<br />b. The internal resistance ‘r’ also increases and equal to ‘nr’. So total resistance of<br />the circuit also increases and is equal to R + nr ohms. Where R – external load<br />resistance.<br />c. Total current is equal to one cell current. That is there is no current increase. If<br />the internal resistance is negligible or less then current will be maximum.<br />Advantages of parallel grouping.<br />a. In parallel grouping emf of one cell will be the total emf of the grouping.<br />b. Total internal resistance of the parallel group is equal to r/n.<br />c. Total resistance of the group is equal to R + r/n.<br />d. Total current = E / (R +r/n) amps.<br />So we can understand that parallel useful when the external resistance is small as<br />compared to internal resistance of the parallel group. But at the same time series<br />grouping is useful when the internal resistance is small compared to the external<br />resistance of the group.<br />112. What are the methods of charging of battery.<br />Mainly there are three types of charging of battery.<br />a. Constant current charging system.<br />In this system the charging current is kept to constant by varying the supplied DC<br />voltage by the help of rheostat or filament lamps in series with the battery, so as to<br />over come the increased back emf of the battery or of the cell.<br />Charging current = V – Eb / R + r amps.<br />b. Constant voltage or potential charging.<br />In this system the voltage is kept to constant, so the charging current in the<br />beginning will be high when the back emf or counter emf of the battery is low and<br />current will be small when the back or counter emf increases as the battery gets<br />charge.<br />c. Trickle charging system.<br />The continuous charging of a battery at a very low rate for keeping the battery<br />ready in good working condition is called the trickle charging. This is to maintain<br />the losses occurring at the idle period. The value of the trickle charging current is<br />approximately 2% of the full charging current of the battery.<br />Question and answers Electrical Maintenance Unit<br />- 34 -<br />113. What are the factors on which the capacity of the battery depends?<br />The capacity of the battery is measured in ampere-hour. The capacity of the battery<br />depends upon the following factors.<br />a. Number and area of the positive plate.<br />b. Discharge voltage. A cell should not be discharged below 1.8 V. If it is<br />discharged below 1.8 V it may cause to reduce the capacity.<br />c. Discharge rate. Capacity decreases with increase rate of discharge.<br />d. Specific gravity of electrolyte. With rapid rate of discharge causes to weaken<br />the electrolyte so the chemical action also weakens and there by the capacity<br />decreases. When the specific gravity increases the capacity of the battery<br />increases.<br />e. Quantity of electrolyte. Electrolyte level should be at the top plate level.<br />f. The design of separator. The design of the separator should be thin.<br />g. Temperature. When the temperature increases the resistance of the battery<br />decreases and the capacity increases.<br />114. Explain Kirchhoff’s laws.<br />Kirchhoff’s laws are used in complex network circuits to determine the equivalent<br />total resistance and the current flowing in various conductors of that circuit.<br />Mainly there are two laws.<br />a. Point law or current law.<br />b. Mesh law or voltage law.<br />Point law or current law.<br />The point law states that, the algebraic sum of the currents meeting at any point or<br />junction or node of a network is zero. In other words the sum of the currents<br />flowing towards the junction or node or any point of network is equal to the total<br />current flowing away from that junction.<br />Mesh law or voltage law.<br />The mesh law states that, in any closed electrical circuit the algebraic sum of the<br />potential drops is equal to the sum of the impressed emf’s acting in that close<br />circuit. In this the important factor is to determine the emf sign to calculate the total<br />emf.<br />115. What are the types of wiring?<br />Mainly there are two types of wiring systems.<br />a. Tree system.<br />b. Distribution system.<br />116. What are the systems of wiring?<br />Following are the general systems of domestic wiring and industrial wiring.<br />a. Cleat system wiring.<br />b. Casing and capping system wiring.<br />Question and answers Electrical Maintenance Unit<br />- 35 -<br />c. Lead sheathed system wiring.<br />d. C.T.S, T.R.S, P.V.C sheathed system wiring.<br />e. Conduit system wiring.<br />Question and answers Electrical Maintenance Unit<br />- 36 -<br />117. What are the testing of wiring installation?<br />Following are the tests to be done after installation of wiring.<br />a. Polarity test.<br />b. Short circuit test.<br />c. Continuity test.<br />d. Insulation test between conductors and conductors to the earth.<br />e. Earth continuity test.<br />118. What are the advantages of AC over DC?<br />a. For the same capacity alternators are cheaper than DC generators, because<br />alternator is not having commutator arrangement and there by small in size.<br />b. Alternating current produces pulsating magnetic field and there by it posses the<br />property of inductance and capacitance.<br />c. Alternating current can be step-up or step-down by static transformer.<br />d. AC can be transmitted with very less cost in comparing to DC transmission.<br />e. Alternating line losses are very less comparing to DC line losses.<br />f. An alternators and AC motor requires very less maintenance.<br />g. Charge per unit for AC is less than DC.<br />119. Define AC.<br />Alternating current is that type of electric current, which changes it’s magnetude<br />and direction periodically.<br />120. What is cycle?<br />One complete set of changes in value and direction of alternating quantity and emf<br />or current is called a cycle.<br />121. What is periodic time?<br />Periodic time is the time taken to complete on cycle. Its symbol is ‘T’. For example<br />Indian standard frequency is 50 cycles per second. So the periodic time T = 1/50<br />seconds. That is equal to 20 m seconds.<br />122. What is frequency?<br />Number of cycles per second is called frequency.<br />123. What is amplitude value or peak value?<br />It is the maximum value of an alternating quantity that can be obtained in any one<br />direction.<br />124. What is instantaneous value?<br />The value of an alternating quantity at a particular instant is called instantaneous<br />value.<br />Question and answers Electrical Maintenance Unit<br />- 37 -<br />125. What is average value or mean value?<br />Average of all instantaneous values of emf or current over a half cycle is known as<br />average value or mean value.<br />Average value = 0.637 * Emax or Imax<br />126. What is root mean square value (R.M.S)?<br />The R.M.S value is also known as effective value or virtual value. The<br />instantaneous value of both the directions will all be squared up and will be added<br />together. Then divide to get the average with the number of instantaneous values<br />and find the square root of this average to calculate the R.M.S value of the emf or<br />current.<br />Or<br />The R.M.S value of an alternating current or emf is equal to the same value of<br />direct current (DC), which produces the same amount of heat with the same time<br />when applied the DC through the same circuit as AC is produced.<br />R.M.S value = maximum value / √2 = 1/√2 = 0.707.<br />∴ R.M.S value or effective value = 0.707 * Emax or Imax<br />127. What is form factor?<br />The ratio of the R.M.S value to the average value is called the form factor.<br />∴ Form factor = 0.707 * Emax or Imax : 0.637 * Emax or Imax<br />= 0.707 * Emax or Imax / 0.637 * Emax or Imax<br />= 1.11<br />So that R.M.S value = average value * 1.11<br />Or average value = R.M.S value / 1.11<br />128. What is crest factor or peak factor?<br />The ratio of maximum value to the R.M.S value is known as crest factor. So the<br />crest factor = maximum value / R.M.S value.<br />= Emax or Imax / (Emax or Imax / √2)<br />= Emax or Imax * √2 / Emax or Imax = √2 = 1.414<br />129. What is vector quantity and what is scalar quantity?<br />Vector quantity<br />A quantity, which has both the direction and magnitude is said to be a vector<br />quantity. Examples are force, emf, current etc.<br />Scalar quantity<br />A scalar quantity is that, which has only magnitude but no direction. Examples are<br />temperature, mass, volume etc.<br />130. What is phase?<br />Question and answers Electrical Maintenance Unit<br />- 38 -<br />The development of an AC quantity through different stages is known as Phase.<br />The term phase refers to the number of separate individual voltage setup in an AC<br />circuit.<br />131. What is in-phase?<br />When those two vectors (voltage and current) attain (reaches) their maximum and<br />minimum values simultaneously (at the same time), then those two quantities are<br />said in-phase. Here between those quantities there is no angle.<br />Question and answers Electrical Maintenance Unit<br />- 39 -<br />132. What is out of phase?<br />When two alternating quantities voltage and current do not reaches their maximum<br />and minimum values simultaneously, then they are called out of phase.<br />133. What is phase angle?<br />Phase angle is an angular displacement between two alternating quantities. Phase<br />angle is measured in electrical degrees or radians.<br />134. What is quadrature quantity?<br />When the phase angle between two vectors is 90° electrical, then they are said to be<br />quadrature quantity.<br />135. What anti-phase quantity?<br />When two quantities are out of phase by 180° electrical, then they are said to be<br />anti-phase quantities.<br />136. What is leading quantity?<br />The alternating quantity that reaches its maximum value earlier than the other<br />quantity is known as the leading quantity.<br />137. What is lagging quantity?<br />The alternating quantity that attains its maximum value later than the other quantity<br />is called the lagging quantity.<br />138. What is the relation between voltage and current in AC circuit containing only<br />resistance?<br />Current (I) is in-phase with the voltage.<br />I = V/R amps.<br />P = I * V * cosϕ or I2 R watts. (Where cosϕ is zero because the voltage and current<br />are in-phase. So cosϕ 0° (zero) = 1)<br />139. What is the relation between voltage and current in AC circuit containing only<br />inductance?<br />Current (I) is lags behind the voltage by 90°.<br />I = V/XL amps.<br />XL = 2πfL ohms.<br />P = I * V * cosϕ watts. (Where cosϕ is 90 because current lags behind voltage by<br />90°. So cosϕ 90° = 0)<br />∴ P = I * V * 0 = 0 watts.<br />Question and answers Electrical Maintenance Unit<br />- 40 -<br />140. What is the relation between voltage and current in AC circuit containing only<br />capacitance?<br />Current (I) is leading the voltage by 90°.<br />I = V/XC amps.<br />XC = 1/2πfC ohms.<br />P = I * V * cosϕ watts. (Where cosϕ is 90 because current is leading the voltage by<br />90°. So cosϕ 90° = 0)<br />∴ P = I * V * 0 = 0 watts.<br />141. What is inductance and inductive reactance?<br />Inductance<br />A coil carrying alternating current produces an alternating flux, which causes to<br />link with same coil and produces an emf in the coil, which opposes the applied<br />emf. This property is known as inductance. The unit for measurement is henry.<br />Inductive reactance<br />The opposition or the reactance offered by the property of inductance in the circuit<br />is known as inductive reactance and denoted by the letter XL. The unit for<br />measurement is ohm.<br />142. What is capacitance and capacitive reactance?<br />Capacitance<br />The property of a capacitor to store electrical energy in it, when it is connected to<br />an electric supply is called capacitance. Unit for measurement is farad. Capacitor<br />store an electric energy in the unit of charge and the unit of charge is coulomb.<br />Capacitive reactance<br />The opposition due to capacitance of capacitor in an electric circuit is called<br />capacitive reactance and it denoted by the letter XC. The unit for measurement is<br />ohm.<br />143. What is impedance?<br />The total opposition offered by an AC circuit for the flow of current through it is<br />called Impedance. The letter ‘Z’ denotes it and the unit is ohm.<br />∴ Z = √ R2 + (XL ∼ XC) 2<br />Z = √ R2 + (X) 2<br />Where ∼ indicates the difference of XL and XC and denoted in the letter X (net<br />reactance of the AC circuit).<br />144. What is ohm’s law for AC circuit?<br />I = V/Z amps.<br />Z = V/I ohms.<br />V = I * Z volts.<br />Question and answers Electrical Maintenance Unit<br />- 41 -<br />Question and answers Electrical Maintenance Unit<br />- 42 -<br />145. What is the current and power in an AC circuit?<br />Current<br />AC circuit contains resistance ‘R’ and reactance ‘X’.<br />In resistive circuit IR = I cosϕ. Because resistance current (IR) is in-phase with<br />voltage (ER).<br />In reactance circuit IX = I sinϕ. Because reactance current will lead or lag the<br />voltage (ER) by 90°.<br />So the resultant current (I) is the vector sum of I cosϕ and I sinϕ. So that circuit<br />current I = √ (I cosϕ)2 + (I sinϕ)2 amps.<br />I cosϕ is some times known as power component of current or the power current or<br />energy current and the I sinϕ is known as reactive component of current or wattless<br />current. Because I sinϕ is not taking any energy from the circuit.<br />Power<br />Power in watts = terminal voltage * power component of current.<br />a. True power = E * I * cosϕ watts.<br />This true power is some times known as energy component or active<br />component or watt-full component. Because this is the power used to produce<br />torque in motor and supplies heat, light etc. or this true power is the power<br />consumption of all source of electric circuit.<br />b. Reactive power = E * I * sinϕ watts.<br />This reactive power is some times known as reactive or in-active component or<br />watt less component or VARS.<br />c. Apparent power = E * I watts.<br />The terminal voltage multiplied by the actual resultant current (I) is called the<br />apparent power or volt-ampere or VA.<br />146. What is power factor?<br />So from the above power explanation,<br />Cosϕ = true power / apparent power = E * I * cosϕ / E * I.<br />So that power factor is equal to<br />a. Cosine of angle of lead and lag of the resultant current with the applied voltage.<br />b. The ratio of R/Z.<br />c. The ratio of true power to the apparent power.<br />Question and answers Electrical Maintenance Unit<br />- 43 -<br />147. What is resonance in series circuit?<br />If in an AC circuit inductive reactance XL and capacitive reactance XC is equal the<br />voltage across both will be equal and are 180° out of phase. So that each will<br />cancel each other and the current limiting component will be the resistance of the<br />circuit.<br />If we are in a position to alter the frequency of supply voltage at a particular<br />frequency named as ‘resonant frequency’, AC series circuit’s XL = XC and the net<br />reactance will be zero. So the current in the circuit is in-phase with the voltage.<br />Because the controlling component of the circuit is resistance only and the current<br />is maximum and equal to V/R amps.<br />This above said condition is called ‘series resonance’ and the frequency at which it<br />occurs is called resonant frequency and the resonant frequency (FR) is equal to<br />(FR) = 1/2π√LC cycles per second.<br />148. What is Q-factor?<br />The ratio of VL/V or VC/V at the resonant frequency is called the voltage<br />magnification denoted as Q-factor.<br />Q-factor = 1 √L/C<br />R<br />149. What is Admittance?<br />Admittance: Admittance is the reciprocal of impedance. It is denoted by the letter<br />‘Y’ and the unit of measurement is mho.<br />Y = I/E = RMS current / RMS voltage.<br />Equation used in admittance<br />a. Conductance ‘G’ = Y * cosϕ = 1/Z *R/Z = R/Z2 mho.<br />b. Susceptance ‘B’ = Y * sinϕ = 1/Z * X/Z = X/Z2 mho.<br />c. Admittance ‘Y’ = √G2 + B2 mho.<br />d. In special cases when X = zero, then G = 1/R and R = zero, then B = 1/X.<br />150. What is the resonance frequency equation for parallel circuit?<br />In parallel circuit when XC = XL, the circuit is called the parallel resonance circuit.<br />That is 2πfL = 1/2πfC.<br />In term (FR) = 1/2π√1/LC – R2/L2 cycles per second.<br />If ‘R’ is negligible, then (FR) = 1/2π√LC cycles per second.<br />151. What is poly phase?<br />A system with two or more the two phases is known as poly phase system.<br />152. What is phase sequence?<br />Question and answers Electrical Maintenance Unit<br />- 44 -<br />The sequence of attaining the maximum value of the induced emf in each set of<br />winding among those three sets is known as phase sequence. This phase sequence<br />is usually indicated by the letters R, Y, B.<br />153. What is phase voltage?<br />The voltage between one of the phase and neutral is known as phase voltage and it<br />is denoted by VPh.<br />Question and answers Electrical Maintenance Unit<br />- 45 -<br />154. What is line voltage?<br />The voltage across any two phases of the supply system is called line voltage and it<br />is denoted by the letter VL.<br />155. What is phase current?<br />The current flowing through any of the phase winding is known as phase current<br />and it is denoted by IPh.<br />156. What is line current?<br />The current flowing between any two phases of the winding is called line current<br />and it is denoted by the letter IL.<br />157. What is balanced load and unbalance load?<br />Balanced load<br />In a three-phase system the power factors and the phase current or line currents of<br />the 3-phase are equal, then that load is called balanced load.<br />Unbalance load<br />If the three-phases have different power factors and the phase current, then the load<br />is called the unbalance load.<br />158. What is phase power and total power?<br />Phase power<br />The power measured between a phase and neutral is known as phase power.<br />Total power<br />The total power measured between the three phases is called total power.<br />159. What are the methods of connecting 3-phase windings?<br />There are two methods.<br />a. Star or wye (Y) connection.<br />b. Delta or mesh (<) connection.<br />160. What are the value of voltage and current in star connection and in delta<br />connection?<br />Star connection<br />a. IL = IPh.<br />b. VL = √3 VPh. ∴ VPh = VL/√3.<br />Note: in star connection we are getting neutral point and we can able to measure<br />the phase as well as line voltage.<br />Delta connection<br />a. VL = VPh.<br />b. IL = √3 IPh. ∴ IPh = IL/√3.<br />Question and answers Electrical Maintenance Unit<br />- 46 -<br />Question and answers Electrical Maintenance Unit<br />- 47 -<br />161. What is the power in 3-phase supply system?<br />In a single-phase system power ‘P’ = VPh * IPh * cosϕ watts.<br />In 3-ϕ system power ‘P’ = 3 * VPh * IPh * cosϕ watts.<br />In Star connection, IL = IPh and VPh = VL/√3. Substituting the value of IPh and VPh<br />in the above 3-ϕ power equation,<br />P = 3 * VPh * IPh * cosϕ watts.<br />P = 3 * VL/√3 * IL * cosϕ watts.<br />P = √3 * VL * IL * cosϕ watts.<br />In Delta connection, VL = VPh and IPh = IL/√3. Substituting the value of IPh and VPh<br />in the above 3-ϕ power equation,<br />P = 3 * VPh * IPh * cosϕ watts.<br />P = 3 * VL* IL/√3 * cosϕ watts.<br />P = √3 * VL * IL * cosϕ watts.<br />So that the power in three phase supply system whether star connected or delta<br />connected is same and power P = √3 * VL * IL * cosϕ watts.<br />So cosϕ = P/ √3 * VL * IL .<br />162. What are the advantages of rotating field system?<br />a. For rotating field alternators only two slip rings and brush gear assembly are<br />required irrespective of number of phases.<br />b. The DC excitation voltage is low and it is very easy to insulate. This intern<br />reduces the size of the machine.<br />c. Out put current can be taken directly from the fixed terminals on the stator. It is<br />easy to insulate high voltage stationary stator (armature).<br />d. The armature winding can be easily braced to prevent any deformation<br />produced by the mechanical stress set as a result of short circuit current and the<br />high centrifugal brought into play.<br />163. What are the types of alternator?<br />Depending upon the speed there are three types.<br />a. Low speed. b. Medium speed. c. High speed.<br />Depending on rotation there are two types.<br />a. Armature rotating b. Field rotating.<br />Depending on number of phases there are two types.<br />a. Single phase b. Poly phases.<br />With respect to excitation there are two types.<br />a. Self excited b. Separately excited.<br />Question and answers Electrical Maintenance Unit<br />- 48 -<br />Question and answers Electrical Maintenance Unit<br />- 49 -<br />164. What is the voltage equation for alternator?<br />165. What is voltage regulation?<br />166. How alternators are rated?<br />Alternators are rated in kVA.<br />167. What are the losses in an alternator?<br />Losses in alternators are same as DC generator and they are as follows.<br />a. Copper losses includes following losses<br />Armature copper losses (Ia<br />2 ra).<br />Field copper losses (Ife<br />2 rf).<br />Losses in brush.<br />b. Stray losses are as follows<br />Magnetic losses (Iron loss or core loss and pole shoes loss).<br />Mechanical losses includes bearing friction, slip ring friction and friction due to<br />windage.<br />168. When the efficiency of the alternator is maximum or on what factor the efficiency<br />of the alternator depends?<br />Efficiency of an alternator depends on its load power factor for a given load. As<br />the power factor decreases Ia increases and the copper losses increases and thus<br />efficiency decreases. The efficiency for given load is maximum only when the<br />power factor is unity and it decreases as the power factor fall.<br />169. What are the methods of synchronizing?<br />a. Lamp method.<br />Dark lamp method and bright lamp method.<br />b. Synchroscope method.<br />170. What is synchroscope?<br />Synchroscope is an instrument, which shows the phase relationship of emf of the<br />incoming alternator and at the same time it also indicates whether it is running slow<br />or fast. This instrument works on the principle of rotating magnetic fields. It<br />consists of a small motor with rotor and stator. Both wound for two phase. A<br />potential transformer connected to two of the main bus-bar give supply to the stator<br />‘A’ winding and another potential transformer of same type connected to the<br />corresponding terminals on the incoming machine supply to the stator ‘B’ winding.<br />The rotor rotates if the stator resultant flux in the ‘A’ and ‘B’ is different and the<br />exact time of synchronizing is the stand still position of the rotor. That means the<br />both the voltages in winding ‘A’ and ‘B’ are same and there is no resultant flux to<br />rotate the rotor. The speed of the rotor depends on the frequency of the alternator<br />Question and answers Electrical Maintenance Unit<br />- 50 -<br />and is too fast when alternator (incoming machine) speed is more and less when<br />alternator is too slow.<br />Question and answers Electrical Maintenance Unit<br />- 51 -<br />171. What is transformer?<br />Transformer is a static device by which AC power at one voltage in one circuit will<br />be transformed into AC power of same frequency at another (decreased or<br />increased voltage) or same voltage to an another circuit, which is in mutual<br />inductive influence with the previous circuit and it is based on mutual electro<br />magnetic induction.<br />172. What are the purposes or advantages of transformer?<br />Purposes<br />a. Electrical energy may be transmitted economically over long distance by<br />stepping up of voltages to reduce the line losses.<br />b. To distribute the low voltages at consumer side by stepping down the voltages.<br />Advantages<br />a. Transformer is a static machine and losses are very less. There by efficiency is<br />high and about 95 to 98%.<br />b. Practically maintenance is very less.<br />173. What is the working principle of transformer?<br />A transformer works under the principle of mutual electro magnetic induction<br />(Faraday’s laws of Electro-magnetic induction). It says that, when ever a changing<br />flux links with a coil an emf is induced in it and this induced emf is proportional to<br />the rate of change of flux and the number of turns in the coils linking the flux.<br />174. What are the types of transformer core?<br />a. Core type transformer core.<br />b. Shell type transformer core.<br />c. Berry type transformer core.<br />d. Spiral type transformer core.<br />175. What is the transformation ratio in transformer?<br />Equation for transformation ratio is,<br />E2/E1 = N2/N1 = k<br />k = >1 in step up transformer, where secondary turns are more and thus voltage is<br />more to reduce the transmission current.<br />k = <1 in step down transformer, where secondary turns are less than primary and<br />low voltage for consumer use.<br />If we include the current in transformation ration the equation is,<br />E2/E1 = N2/N1 = I1/I2 = k<br />176. What is the use of conservator in the transformer?<br />It is a drum type cylinder mounted on the top of the transformer through a small<br />pipe. … of the conservator is kept empty. To indicate the level of oil in the<br />Question and answers Electrical Maintenance Unit<br />- 52 -<br />transformer an indicator is fixed. Conservator will help the oil inside the tank by<br />providing sufficient space to expand and to contract as its temperature varies<br />without exposing much surface area. That is it limits the air with oil due to its less<br />surface area.<br />Question and answers Electrical Maintenance Unit<br />- 53 -<br />177. What is the use of breather in the transformer?<br />Breather is a bottle shaped steel tube, which is attached to one side of conservator<br />to allow the air to pass in and out of the tank or conservator through the calcium<br />chloride and silica gel, which is filled in it to absorb the moisture contained in the<br />air. When the silica gel absorb the moisture its colour changes from blue to pink.<br />178. What is the use of buchholz relay in the transformer?<br />It’s a protection relay used in oil immersed transformer to protect the transformer<br />from insulation failure, core heating or any other type of internal faults, which may<br />cause the heating of winding beyond the specified temperature. This relay is placed<br />in between the pipe connecting the conservator and the tank. Generally used in<br />power transformer of above 500 kVA.<br />It consists of two operating floats and is operated by two mercury switches<br />separately provided for the alarm and trip. Due to internal fault (collection of gases)<br />or leakage of oil if the oil level comes down the alarm relay first operates and then<br />the trip relay operates to isolate the transformer from the circuit.<br />179. What is the use of explosion vent in the transformer?<br />It is also a safety device of a transformer, which protects the transformer tank from<br />the high consequences of the high-pressure gases induced or developed by any type<br />of short circuit in the transformer by allowing the gas to escape by puncturing the<br />diaphragm.<br />180. What is the emf equation for transformer?<br />Always maximum flux reaches from zero to maximum in one quarter of the cycle.<br />That is in … of second. That is equal to 1/200 second.<br />Average rate of change of flux = Qm / … f. = Qm * 4 * f.<br />= 4 f Qm Weber / second.<br />As the coil has N turns the average emf induced in the coil = 4 f Qm N volts.<br />But the rms. Value = average value * form factor.<br />∴ rms. Value of emf = 1.11 * 4 f Qm N volts.<br />= 4.44 f Qm N volts.<br />181. What are the losses in transformer?<br />In transformer there are losses due to,<br />1. Resistance of the winding (copper losses).<br />2. Eddy current and Hysterisis in the iron parts and core (core and iron losses)<br />3. Losses due to leakage reactance (leakage flux).<br />At No load the copper losses and leakage flux losses are negligible due to the very<br />less primary current.<br />At loaded condition copper losses and leakage flux losses will exist in cosiderable<br />manner. Copper losses are variable and can be calculated by Ip<br />2*rp and Is<br />2*rs.<br />Question and answers Electrical Maintenance Unit<br />- 54 -<br />Question and answers Electrical Maintenance Unit<br />- 55 -<br />182. What are the types of cooling in transformer?<br />1. Natural cooling.<br />a. Air natural cooling (Dry type).<br />b. Oil immersed natural cooling.<br />c. Oil immersed, forced oil circulation with natural cooling.<br />2. Artificial cooling.<br />a. Oil immersed forced air circulation with air blast cooling.<br />b. Oil immersed blast cooling.<br />c. Air blast cooling.<br />3. Artificial cooling (water).<br />a. Oil immersed water cooling.<br />b. Oil immersed forced oil circulation with water cooling.<br />4. Mixed cooling (water).<br />This is the method of cooling combining oil natural, water, air natural, air blast<br />and forced oil.<br />Question and answers Electrical Maintenance Unit<br />- 56 -<br />183. State the type transformers?<br />Transformers can be classified into different groups and types based on the<br />following factors.<br />1. Type of core.<br />a. Core type transformer core.<br />b. Shell type transformer core.<br />c. Berry type transformer core.<br />d. Spiral type transformer core.<br />2. Method of cooling.<br />a. Natural cooling transformer.<br />b. Artificial cooling transformer.<br />c. Artificial cooling (water) transformer.<br />d. Mixed cooling transformer.<br />3. As per transformer ratio.<br />a. One to one transformer.<br />b. Step down transformer.<br />c. Step up transformer.<br />4. Based on number of phases.<br />a. Single-phase transformer.<br />b. Two-phase transformer.<br />c. Three phase transformer.<br />5. As per winding connection.<br />a. Star-star connected.<br />b. Star-delta connected.<br />c. Delta-delta connected.<br />d. Delta-star connected.<br />e. Open delta connected.<br />f. Scott connected.<br />6. As per the size of the transformer.<br />a. Distribution transformer (upto 500 kVA).<br />b. Power transformer (above 500 kVA).<br />7. Based on function and utilization.<br />a. Auto transformer.<br />b. Potential transformer (instrument transformer).<br />c. Current transformer (instrument transformer).<br />184. What is the humming of transformer?<br />Humming is a sound, which is produced due to the vibration of the cores in the<br />transformer. The vibrations are produced due to the change in polarity of an<br />alternating current or voltage and by the loose of lamination of the core. Both can<br />be minimised by tightening the core of the transformer.<br />Question and answers Electrical Maintenance Unit<br />- 57 -<br />185. What are the types of AC three phase motors?<br />Mainly there are two types.<br />1. Synchronous motors.<br />a. Plain synchronous motors.<br />b. Auto synchronous motors.<br />2. A-synchronous motors.<br />a. Induction motors.<br />1. Single phase motors<br />• Shaded pole motor.<br />• Capacitor start capacitors run motor.<br />• Capacitor start induction’s run motor.<br />• Split face motor.<br />2. Three phase motors.<br />• 3φ single squirrel cage motor.<br />• 3φ double squirrel cage motor.<br />• Squirrel deep bar induction motor.<br />• Slipring induction motor.<br />b. Commutator motors.<br />1. Single phase commutator motors.<br />• Plain repulsion motor.<br />• Repulsion start induction’s run motor.<br />• Repulsion induction motor.<br />• Series motor or universal motor.<br />2. 3φ commutator motors.<br />• 3φ series motor<br />• Charge motor.<br />• Compensated motor.<br />186. What is the working principle of 3φ induction motor?<br />When 3φ supply is given to stator, a rotating magnetic field of constant magnitude<br />is produced. This rotating magnetic field produces induced emf in the rotor<br />winding as per faraday’s laws and this induced emf causes to circulate a heavy<br />induced current in the rotor winding due to very small resistance of rotor. At the<br />initial moment the frequency of induced emf is equal to the frequency of the stator<br />supply voltage, when the rotor is stationary as in the case of secondary of a<br />transformer. The rotor induced current according to lenz’s law flows in such a<br />direction that it opposes the cause, which is inducing it. In this case the cause<br />producing the rotor current is the relative speed between the rotating magnetic field<br />if stator and the rotor and is maximum when the rotor is stationary. Hence to reduce<br />this relative speed rotor conductor (rotor) starts to rotate in the same direction in<br />Question and answers Electrical Maintenance Unit<br />- 58 -<br />which the stator field is rotating and tries to catch it up. The rotation of this rotor is<br />developed due tog the torque developed in the rotor by interaction between the<br />rotating magnetic field of stator and the field produced by the rotor current.<br />Question and answers Electrical Maintenance Unit<br />- 59 -<br />187. What is torque?<br />As said above torque is a turning or twisting moment of a force about an axis and it<br />is measured by the product of force * radius at which the force acts.<br />There are two types of torques.<br />a. Starting torque: This is the torque, which is required to start the motor at<br />load or no-load.<br />b. Running torque: This is the torque, which is required to run the motor at<br />normal speed and at normal load.<br />The letter ‘T’ denotes torque in induction motor and torque is proportional to<br />Ir φ cosϕr.<br />That is T ∝ Ir φ cosϕr. Where Ir = rotor current.<br />φ = Flux = stator flux per pole in Weber.<br />Cosϕr = rotor power factor.<br />188. What is slip?<br />The difference in speed of stator magnetic speed ‘Ns’<br />(synchronous speed) and rotor speed ‘Nr’ is called slip<br />or absolute slip and it is denoted by the letter ‘S’.<br />∴ S = Ns – Nr / Ns.<br />Slip has no unit. Percentage of slip of induction<br />motor varies from 4 to 5% in small motors and 1.5 to<br />2.5% in big motors.<br />In other words slip ‘S’ = fr / f. Where fr is rotor<br />frequency and f is stator frequency.<br />189. What is the working principle of double squirrel cage<br />induction motor?<br />In double squirrel cage motor outer cage rotor winding<br />is of high resistance and low reactance. Inner cage<br />winding is of high reactance and low resistance.<br />At the time of starting rotor frequency is equal to<br />the stator frequency and there by the reactance of the<br />inner cage winding is comparatively high (XL = 2πfL) because<br />it is linking more inner winding than the outer winding. So the impedance of inner<br />cage winding is very high. Hence the current flow through inner cage winding is<br />very less comparing to the outer cage winding. That is a very high ratio of current<br />is passing through the outer cage winding at the time of starting and there by<br />produces very high starting torque.<br />When the rotor starts running the speed of the motor can be increased and the slip<br />will be decreased and there by the rotor frequency (‘S’ = fr / f). So that in<br />the running condition the reactance of the inner cage decreases to the lowest value<br />and hence the Impedance (XL = 2πfL). So the current in inner cage winding will be<br />comparatively more than the outer cage winding at the time of running. So now<br />Question and answers Electrical Maintenance Unit<br />- 60 -<br />inner cage winding produces more torque than outer cage at the time of running<br />and the motor running torque is good enough.<br />Question and answers Electrical Maintenance Unit<br />- 61 -<br />190. Why starter is necessary to start the AC motor?<br />a. At the time of starting motor starting current is<br />high (4 to 5 times). Therefore if motor is directly<br />started the supply voltage may be disturb.<br />b. By the help of starters starting and stopping of<br />motors can be made easily as we required. Because<br />starters provides overload tripping difficulties.<br />c. The help of starters can protect motor against the<br />single phasing by the action of overload<br />arrangements.<br />d. Protect the motor from no-voltage and its<br />difficulties.<br />e. Permits automatic control when required.<br />191. What are the types starters used for starting of<br />induction motor?<br />a. Direct on line starter (air break) mechanically.<br />b. Direct on line starter (air break or oil immersed)<br />electrically.<br />c. Star delta starter.<br />d. Slipring motor starter.<br />e. Auto transformer starter.<br />192. What are the speed control methods of induction motor?<br />a. By controlling the supply voltage.<br />b. By controlling the supply frequency (Ns = 120f / P).<br />c. By varying the number of poles (Ns = 120f / P).<br />d. By rotor rheostatic control (for small speed<br />variation).<br />193. What is magnetic locking or cogging effect of<br />induction motor?<br />In squirrel cage induction motor some times the rotor<br />and stator care teeth or slots are comes face to face<br />or parallel at stationary condition. If we are starting<br />the motor at this condition the motor get hesitated to<br />start or run due to the attraction developed between<br />those rotor and stator teeth or slots. This is known as<br />the magnetic locking or cogging effect of a squirrel<br />cage induction motor. This type of magnetic locking in<br />squirrel cage induction motor can be avoided either by<br />skewing the rotor slot or by selecting the rotor slot,<br />such that there is no common factor between the rotor<br />slot and stator slots.<br />194. What is skewing?<br />Skewing can be done by turning the rotor slots about 15°<br />from the parallel position of slots with the shaft.<br />Question and answers Electrical Maintenance Unit<br />- 62 -<br />That is rotor slots are not in parallel with the shaft<br />but there is an angle of about 15° with the shaft.<br />195. What are the losses in induction motor?<br />a. Stator losses (stator copper losses, stator iron<br />losses).<br />b. Rotor losses (rotor copper losses, rotor iron<br />losses).<br />c. Windage and friction losses.<br />196. What is synchronous motor?<br />An alternator, which is running as a motor can be<br />called as synchronous motor and it runs at synchronous<br />speed while it converts electrical energy into<br />mechanical energy. It requires both AC for armature and<br />DC supply for field.<br />Question and answers Electrical Maintenance Unit<br />- 63 -<br />197. What are the advantages and dis-advantages of<br />synchronous motor?<br />Advantages<br />a. It’s a constant speed motor and is equal to<br />synchronous speed from no load to full load.<br />b. It has good efficiency higher than induction motor.<br />c. It can be run as a motor and also as an alternator as<br />per the requirement. More over it can be used as<br />synchronous condenser.<br />Dis-advantages<br />a. It can not be used as a varying speed motor. Because<br />its speed can not be varied.<br />b. As a motor it is not self-starting type and it can<br />not be started on load.<br />c. It requires both AC and DC supply.<br />d. Hunting is also produced in this motor.<br />198. What are the applications of synchronous motor?<br />a. These motors are used in powerhouses, in sub stations<br />for the improvement of power factor by connecting it<br />in parallel to the supply and it is run without load<br />under over excitation of field.<br />b. Used in big industries where many induction motors<br />are installed to improve the power factor.<br />c. Used for constant mechanical loads.<br />199. What is hunting effect?<br />When the load is varied to the motor the oscillation<br />being setup in the rotor about the position of<br />equilibrium corresponding to change of load condition.<br />So the damper winding acts the magnetic lines of force<br />and causes to create the opposite torque, which keeps<br />the rotor in the same position of the particular load.<br />This oscillation of the rotor is known as Hunting or<br />Phase swinging. To reduce this hunting damper winding<br />is helpful.<br />200. What is synchronous condenser or phase advancer?<br />An over excited synchronous motor takes leading current<br />just like a condenser and gives leading power factor. A<br />synchronous motor, which I used only for the purpose of<br />improving power factor, can be called as synchronous<br />condenser or phase advancer.<br />201. Why single-phase motors are not self-starting?<br />When a 1φ supply is given to the single winding of the<br />single phase motor, the field produced by it changes in<br />magnitude and direction sinusoidally (pulsating flux).<br />Question and answers Electrical Maintenance Unit<br />- 64 -<br />Such and alternating field is equivalent to two fields<br />of equal magnitude and speed rotating in opposite<br />direction. Such rotating magnetic fields produces two<br />torque’s on the rotor. So the rotor can not rotate in<br />any direction. Because the net torque developed by the<br />motor is equal to zero. So a single-phase motor is not<br />self-starting.<br />Question and answers Electrical Maintenance Unit<br />- 65 -<br />202. What are the methods to self-starting of single-phase<br />motor?<br />a. Splitting one phase into two phases.<br />b. By using capacitor.<br />c. By using repulsion method.<br />d. By shading the poles.<br />e. By connecting the field in series with the rotor<br />having winding with commutator (AC series motor or<br />universal motor).<br />203. What are the methods to control the speed of singlephase<br />motor?<br />In AC single-phase motors speed control can not be<br />achieve as smooth as in DC motor. There are following<br />few methods of speed control.<br />a. By changing the number of poles of stator.<br />b. By changing the applied voltage to the stator.<br />c. Frequency control method.<br />d. Rotor rheostat control.<br />e. By operating two motors in concatenation or cascade<br />or tandem method.<br />f. By injecting an emf in the rotor circuit.<br />g. By changing slip.<br />204. What are the classifications of electrical measuring<br />instruments?<br />a. Absolute instruments. These instruments give the<br />value of the quantity to be measure in terms of the<br />constant of the instrument and their deflection only.<br />There is no any calibrated scale.<br />b. Secondary instruments. Secondary instruments are<br />those, which are calibrated in comparison with some<br />absolute instrument so as to indicate the electrical<br />quantity to be measured with the deflection of needle<br />or pointer of that meter over a calibrated scale.<br />205. What are the operating principles of electrical<br />measuring instruments?<br />a. Magnetic effect.<br />b. Electro dynamic effect.<br />c. Electro magnetic effect.<br />d. Thermal effect.<br />e. Chemical effect.<br />f. Electro static effect.<br />206. What are the classifications of secondary instruments?<br />a. Indicating instruments.<br />b. Recording instruments.<br />Question and answers Electrical Maintenance Unit<br />- 66 -<br />c. Integrating instruments.<br />207. What are the essentials of indicating instrument?<br />a. Deflecting torque or force (effect of electricity).<br />b. Controlling torque or force (spring control and<br />gravity control).<br />c. Damping torque or force (air friction, eddy current<br />and fluid friction).<br />Question and answers Electrical Maintenance Unit<br />- 67 -<br />208. What are the possible errors in induction (energy<br />meter) measuring instruments?<br />a. Phase error: Field flux in induction meter does not<br />lag 90° behind the supply voltage due to its<br />resistance. This can be adjusted by copper shading<br />rings, which are placed at the central limb of the<br />shunt magnet.<br />b. Speed error: An error in speed, which is tested on<br />the non-inductive load, can be eliminated by<br />correctly adjusting the position of the brake magnet.<br />c. Friction error: It can be reduced very much by<br />providing two copper shading st the both outer limbs.<br />d. Creeping error: Some time slow, continuous rotation<br />of the disc (rotor) when only the pressure coil is<br />excited, but no current flowing in the circuit (no<br />current in current coil) may happen. It may be caused<br />due to incorrect friction compensator, stray magnetic<br />field, and excess voltage. This can be rectified by<br />drilling two holes in the disc on the opposite sides<br />of the spindle. This causes sufficient distortion of<br />the field to prevent rotation, when one of the holes<br />comes under one of the pole of the shunt magnet.<br />209. What is illumination?<br />The quantity of a light emitted by a lighting source is<br />known as illumination. Heating effect of electric<br />current is used in producing illumination. When a solid<br />or vapour is heated it begins to radiate energy in the<br />surrounding media.<br />Lux is the unit for illumination. Lux is the<br />illumination produced by a uniform source of candle<br />power on the inner surface of a sphere of radius one<br />(1) meter.<br />210. What are the laws of illumination?<br />a. Illumination ‘E’ is directly proportional to the<br />luminous intensity ‘I’ of the source. ie E ∝ I.<br />b. Inverse square law: The illumination of the surface<br />is inversely proportional to the square of the<br />distance of the surface from the source. ie E ∝ 1/d2.<br />c. Illumination ‘E’ is directly proportional to the<br />cosine of angle made by the normal to the<br />illumination surface and the direction of the<br />incident light and is known as lam pod’s cosine<br />angle.<br />Question and answers Electrical Maintenance Unit<br />- 68 -<br />211. What are the factors to be considered for correct<br />illumination?<br />a. Nature of work.<br />b. Determine the foot-candle illumination required after<br />studying the nature of work. Example for precision<br />work – 100 foot candle, for fine engraving – 50 foot<br />candle, for reading, typing, drawing, fine machine<br />works 25 foot candle etc.<br />c. Design of apartment using for the proper projection<br />of illumination for better work or purpose.<br />212. What are the types of lighting?<br />a. Direct lighting: light directly comes from the source<br />to the surface.<br />b. Indirect lighting: light reflects from the wall,<br />reflector or ceilings etc.<br />c. Semi direct lighting: light comes through the shade.<br />Question and answers Electrical Maintenance Unit<br />- 69 -<br />213. What are the properties of good illumination?<br />a. It should have sufficient light.<br />b. It should not strike the eyes.<br />c. It should not produce glares.<br />d. Light should be uniform.<br />e. It should be of harmonious.<br />f. It should be of correct type as needed.<br />g. It should have suitable shade and reflector.<br />h. Economically productive.<br />214. What are the sources of light?<br />a. Incandescent lamps.<br />b. Carbon arc lamps.<br />c. Gas discharge lamps.<br />215. What are the materials used in Neon sign tube lamps<br />for different colors?<br />Following are the materials used in neon sign tubes for<br />different colors.<br />For, Red – Neon gas.<br />Reddish orange – Neon gas + Argon gas.<br />Blue – Vapour of mercury.<br />Golden – Neon gas + Helium gas.<br />Green – mixture of Neon gas and mercury in yellow<br />glass tube.<br />By depositing fluorescent powder on the inner surface<br />of the tube varying colors in intensity can be made.<br />216. What is the material used in florescent tube?<br />The fluorescent tube is filled with argon gas at law<br />pressure and some mercury after evacuating the tube.<br />This argon gas gives initial starting at quick manner.<br />Initially the mercury is in the form of globules on the<br />inside of the tube surface. As the temperature<br />increases the liquid takes globules mercury changes<br />into vapour form and takes over the conduction of the<br />current.<br />217. What are the importances of conversion of AC into DC?<br />a. For traction purpose a DC series motor is most<br />important. Examples in railways, in electrical lifts<br />etc.<br />b. For electrolytic and electro chemical processes such<br />as electro plating, electrolysis, electro refining<br />only DC is essential.<br />c. DC is essential for battery charging, running arc<br />lamp torch, cinema projector and for arc welding.<br />Question and answers Electrical Maintenance Unit<br />- 70 -<br />d. It is required for operating relays, timer,<br />telephone, circuit breakers etc.<br />Question and answers Electrical Maintenance Unit<br />- 71 -<br />218. What is rectifier? Write few types of rectifiers.<br />Rectifier is a device which converts AC supply into DC.<br />Following are the types of rectifiers generally used.<br />a. Copper oxide rectifier.<br />b. Selenium rectifier.<br />c. Electrolytic rectifier.<br />d. Mercury arc rectifier.<br />e. Tungar rectifier.<br />219. What are the parts of paper insulated lead covered<br />cable?<br />a. Core.<br />b. Insulation of cable.<br />c. Metallic sheath.<br />d. Bedding.<br />e. Armouring.<br />f. Serving.<br />220. What are the factors considered for selecting a cable?<br />Following factors considered for the selection of the<br />cable.<br />a. System voltage.<br />b. Condition of installation.<br />c. Continuous current to be carried.<br />d. Maximum operating conductor temperature (70°).<br />e. Ambient air temperature (40°).<br />f. Thermal resistivity of soil.<br />g. Depth of laying.<br />h. Short circuit current. Ish = Ka / (t/2). Where ‘K’ is<br />constant (K = 109 for copper cables), ‘a’ is area and<br />‘t’ is time duration of short circuit in seconds.<br />221. What are the advantages of high voltage transmission?<br />a. Saving in conductor materials.<br />b. Low power loss (I2R) of transmission lines due to<br />decrease in current.<br />c. Better efficiency of line due to fewer losses.<br />d. Better voltage regulation due to less voltage drop in<br />line due to less transmission current.<br />e. Due to the less cross section of conductor distance<br />between the poles increases and the cost decreases<br />and the labour cost also decreases.<br />222. What are the types of distribution system?<br />a. Radial distribution system.<br />b. Ring distribution system.<br />c. Grid distribution system.<br />Question and answers Electrical Maintenance Unit<br />- 72 -<br />223. What are types of distribution of supply?<br />a. Over head distribution system.<br />b. Under ground distribution system.<br />Question and answers Electrical Maintenance Unit<br />- 73 -<br />224. What are the main items used in over head distribution<br />system?<br />a. Conductor material.<br />b. Pole.<br />c. Cross arm.<br />d. Insulators.<br />e. Strain insulator.<br />f. Post insulator.<br />g. Stay wire.<br />h. Support with insulator and stay lightner.<br />225. Write types of lightning arrestor.<br />a. Horn gap lightning arrestor.<br />b. Oxide film lightning arrestor.<br />c. Pellet lightning arrestor.<br />d. Thyrite lightning arrestor.<br />226.<br />Question and answers Electrical Maintenance Unit<br />- 74 -<br />Motor, Generator and Exciter.<br />1. What are the main classifications of alternator?<br />a. Salient pole.<br />b. Non – salient pole.<br />2. What is the emf equation of alternator?<br />Flux cut per second by each conductor = 2 φm f<br />Average emf generated in each conductor = 2 φm f Z<br />Average emf generated per phase = kd kc 2 φm f Z<br />r.m.s emf generated per phase = kf kd kc 2 φm f Z<br />For sinusoidal waveform when kf is 1.11 then emf generated<br />= 1.11 *2 kd kc φm f Z<br />= 2.22 kd kc φm f Z<br />kf 􀃆form factor<br />f 􀃆 Frequency<br />φm 􀃆 Flux maximum<br />Z 􀃆 Turns per phase<br />kd 􀃆 Breadth factor or distribution factor<br />kc 􀃆 Coil span<br />3. Why conductors in alternator are transposed?<br />To reduce eddy current losses.<br />4. What is the effect of frequency and high voltage at the start of motor.<br />For a constant load if frequency decreases motor current will increase and at the start<br />if voltage is more motor current also increases.<br />5. What is the minimum voltage required for starting of 6.6 kV motors?<br />Minimum 80% of rated voltage.<br />6. What are the limits of vibration measurement for motors?<br />50 microns for displacement and 5 mm/second for velocity.<br />7. What you mean by SPDP?<br />Screen protected drip proof.<br />8. What is the current in single phasing?<br />2 times of rated current.<br />9. What is the impedance per phase of delta connected motor?<br />1.5 times the total impedance.<br />Question and answers Electrical Maintenance Unit<br />- 75 -<br />10. What is the slip of an induction motor during normal running?<br />More than zero.<br />Question and answers Electrical Maintenance Unit<br />- 76 -<br />11. What is the effect of increased load on power factor of induction motor?<br />Power factor of an induction motor increases with loading.<br />12. Explain the behavior of generator when operating alone and operating parallel with<br />grid.<br />Generator operating alone.<br />a) The power factor of generator depends on load power factor.<br />b) The terminal voltage decreases when generator is loaded.<br />c) Governor decides the frequency of generator.<br />d) Increase in excitation increases the terminal voltage of the generator.<br />Generator parallel with grid.<br />a) If we increase the steam input to the generator the frequency of the generator will<br />not change. It will remain practically constant as same as grid frequency. That is<br />grid decides the frequency of the generator.<br />b) Increase in the excitation will not increase the terminal voltage. Instead the<br />reactive power out put of the generator increases. This reactive power supplies for<br />the magnetizing current of motors, transformers and etc.<br />c) Increase in the steam input increases the active power of the generator.<br />d) If generator is under excited it will draw leading reactive current from the grid.<br />13. Draw and explain following.<br />a) Load current Vs terminal voltage at different power factors.<br />1. At leading power factor as the load current increases the terminal voltage also<br />increases.<br />2. At lagging power factor as the load current increases the terminal voltage<br />drops.<br />3. At unity power factor as the load current increases there is slight drop in<br />terminal voltage.<br />Leading power factor<br />Unity power factor<br />Lagging power factor<br />Ter. Vol.<br />Load current<br />Question and answers Electrical Maintenance Unit<br />- 77 -<br />b) Torque Vs slip characteristics of induction motor.<br />Torque (T) = φ I2 cosφ2<br />Where φ - main flux.<br />I2 – rotor current Pull out torque (R = XL) I<br />Cosφ2 – rotor power factor.<br />Starting current<br />Starting torque (1.5 times)<br />Torque Full load current<br />1 Slip 0<br />When motor is started from rest the slip is 1 (one) at time of starting and starting<br />torque is 1.5 times of rated torque. As the motor accelerates slip reduces and torque<br />increases. Because the power factor of rotor is improving due to the decrease in rotor<br />frequency and at certain slip the rotor reactance is equal to rotor resistance. At that<br />time torque is maximum (pull out torque). When the motor accelerates to the rated<br />speed the torque comes to the rated value, which is less than the starting torque.<br />Torque is zero when slip is zero, because there will be no relative motion between<br />stator magnetic field and rotor.<br />c) Generator capability curve<br />This curve gives the operating limits of the turbine generator at different power<br />factor what should the power output of the generator to avoid the heating of<br />generator stator winding, rotor parts and end parts.<br />Question and answers Electrical Maintenance Unit<br />- 78 -<br />Question and answers Electrical Maintenance Unit<br />- 79 -<br />14. What is the cooling medium for different parts of the turbo generator?<br />Stator – DM water.<br />Rotor – Hydrogen.<br />Bearing – Oil.<br />15. What are the effects of unbalanced voltage on induction motor?<br />There will be negative sequence current, which will heat up the stator winding and<br />weakens the insulation. These currents will induce emf in rotor and heat up the rotor<br />bars and cause breakage in them. Also due to high leakage fluxes due to negative<br />phase sequence current the end parts heating will be more.<br />16. What are the types of bearings are adopted for small motors and large motors?<br />Small motors (LT motors)<br />Horizontal mounted – deep groove ball bearing at both ends.<br />Medium motors<br />Roller bearing at DE and deep groove bearing at NDE.<br />Large motors (HT motors above 750 kW)<br />Horizontal mounted – sleeve bearing (pedestal) cooled by lub oil.<br />Vertical mounted – face to face angular contact ball bearing at NDE and roller or ball<br />bearing at DE.<br />Question and answers Electrical Maintenance Unit<br />- 80 -<br />Insulation classification and testing.<br />1. What is good dielectric break down value for insulating oil?<br />60 kV.<br />2. What is the temperature coefficient of insulating materials?<br />Insulators are negative temperature coefficient materials.<br />3. What is gap between the electrodes in transformer oil testing kit?<br />0.1 Inch.<br />4. What is the life insulation if temperature increased by 10°C?<br />The life of the machine insulation decreases by half if the temperature of the<br />insulation increases by 10°C.<br />5. What is the value of vacuum maintained by vacuum pump in oil filteration machine?<br />27 Hg.<br />6. What is the DC HV test voltage range?<br />1.7* 1.5* rated voltage.<br />7. What do you mean by term insulating resistance? How it is measured?<br />Insulating resistance: insulating resistance is the opposition offered by an insulating<br />material to the flow of current (electrons) through it when an high potential is<br />applied across it.<br />Insulating resistance are measured by megger.<br />First the equipment whose resistance is to be measured is disconnected from supply.<br />If the machine is a large one, there may be accumulated static charge on the machine.<br />So we have to discharge it by connecting a wire between the terminals and ground<br />for 15 minutes. Otherwise megger will give wrong reading.<br />After this we should remove the wire and we have to connect megger terminals (live<br />&amp; earth) to the motor terminal and earth. The rating of the megger should be selected<br />properly. Then rotate the megger at rated speed of 160 rpm and take the readings.<br />Question and answers Electrical Maintenance Unit<br />- 81 -<br />8. What you mean by dielectric absorption test?<br />Whenever we apply a potential from the megger to test the IR value, initially the<br />needle of the megger will go to low value of the resistance. This is due the<br />capacitance effect of the insulation material and after some seconds the needle will<br />start moving towards the higher value. Because in the insulating material there is<br />strain on the molecules when the potential is applied. Polarization of the molecules<br />occurs and they form a Di – pole. The negative charges are attracted to positive<br />terminal and positive charges are attracted to negative terminal. So there is a strain<br />on the insulation molecules and they align themselves parallel. This aligning may<br />take more time. This test is done to know the condition of insulating material.<br />I<br />(μ Amps)<br />(A)<br />(B)<br />Time<br />If the insulation is good the graph is as shown as B and if there is dirt, moisture the<br />graph will flatten early as shown in A.<br />After the test terminals to be discharged so that molecules may return to their<br />unstressed state.<br />Question and answers Electrical Maintenance Unit<br />- 82 -<br />9. Draw the transformer drying out curve and explain each stage.<br />At this point the heaters are<br />IR and Switched off.<br />Temp. Temp.<br />IR<br />1st 2nd 3rd<br />Time in hours<br />When we start the filtering process initially the temperature will be low, as the<br />insulation value is high. But on temperature increases the IR value starts to decrease<br />because the moisture entrapped in the coils are released due to rise in temperature<br />and this causes the IR value to go down. This is the first stage.<br />Then comes the point where all the moisture is released and then will be no decrease<br />in IR value or rise in the temperature. This is the second stage.<br />At this point the heaters are switched off. Now the moisture is removed by the oil<br />filters and the IR value goes up and as the heaters are off the temperature decreases.<br />This is the third stage.<br />10. The insulation resistance of a DC motor is observed to be 15 MΩ at a temp. of 70°C.<br />what is its value corrected to 40°C. the correction factor for 70°C is 8.0.<br />Observed resistance at 70°C – 15 MΩ.<br />Temperature correction factor – 8.<br />Rm = kt * Rt kt – correction factor.<br />Rm = 8 * 15 Rt – resistance measured at +°C.<br />Rm = 120 MΩ. Rm – corrected value to 40°C<br />The IR of DC motor corrected to 40°C is 120 MΩ.<br />Question and answers Electrical Maintenance Unit<br />- 83 -<br />11. The armature of a 600 kW, 0.24 k, 1000 rpm DC generator has an indicated IR to<br />ground of 2 MΩ at a temp. of 30°C. what is the recommended value of insulation? Is<br />it advisable to put the machine in service? Give reason. Correction factor for 30°C is<br />0.5.<br />Data given are<br />kV – 0.24<br />Indicated IR – 2MΩ<br />Temp. - 30°C<br />Correction factor – 0.5<br />Recommended value (Rm) = kV + 1 MΩ<br />= 0.24 + 1<br />= 1.24 MΩ<br />Indicated IR at 30°C = 2MΩ<br />Correction factor – 0.5<br />So value corrected to 40°C = Rm = kt * Rt<br />= 0.5 * 2<br />= 1MΩ<br />The generator cannot be put in service because the corrected value is lesser than<br />recommended value. It should be sent for IR re-conditioning .<br />Question and answers Electrical Maintenance Unit<br />- 84 -<br />Motorised valve actuator<br />1. What are the advantages of motorised valve actuators?<br />Advantages<br />a. Can be used to operate in remote areas, high heat areas etc.<br />b. Suitable logics can be wired up easily.<br />c. Hammer blow mechanism (which will release valve in stuck open or close). The<br />motor has high torque.<br />2. What are the four basic parts of any valve actuators?<br />a. Valve motor.<br />b. Gear mechanism.<br />c. Limit switch and torque switch assembly.<br />d. Terminal box.<br />3. Explain how the motor motion is transmitted to the valve stem?<br />The motor shaft is connected to a spar gear. It engages on a worm wheel. The worm<br />wheel has dog teeth. This dog teeth engages or hits the dig teeth on moving or sliding<br />clutch. The sliding clutch has splines and these are on the splines of valve stem. So<br />when the sliding clutch rotates the valve also rotates simultaneously.<br />4. Do you require separate limit switch for closing and separate limit switch for<br />opening? Ans. YES.<br />5. What does the limit switch mechanism senses to operate?<br />Limit switch mechanism senses whether the open and close travel of the motor has<br />exceeded the limit setting of the motor to operate.<br />6. What does the torque switch mechanism senses to operate?<br />Torque switch senses whether the torque of motor has exceeded the set point<br />irrespective of position of valve.<br />7. Explain the operational aspects of limit switch and torque switch in rotork valve<br />actuator.<br />a. When limit function is selected?<br />b. When torque function is selected?<br />Limit function – when limit function is selected in rotork valve, the limit switches<br />will operate when the limit set points are reached. Suppose limit switches fails to<br />operate the torque switches will act and cuts off the supply to the motor (both torque<br />switch and limit switches can act when selected to limit function).<br />Torque function – when torque function is selected the torque switches will act when<br />set point is reached. The limit switches will not act (only torque switch will act when<br />selected to torque function).<br />Question and answers Electrical Maintenance Unit<br />- 85 -<br />8. Indicate how you will select the limit switch contacts for indication and for cutting<br />the supply to the motor?<br />For cutting off the supply to the motor normally closed (NC) contacts of limit switch<br />(LS) should be wired in series with interposing relay coil of respective direction of<br />the valve so that when valve reaches their respective direction contact will open and<br />cut the control supply. For indication normally open contacts (NO) of opposite<br />direction of valve should be used so that when valve fully closes open indication<br />contact should remain NC only and vice-versa.<br />So for close direction<br />NC contact of close direction<br />9. What indication will you get in control room when<br />a) valve is open – green<br />b) valve is closed – amber<br />c) valve is intermediate – both<br />d) valve is closing and torque switch operate – both<br />10. What is the function of hammer blow mechanism?<br />Hammer blow mechanism allows motor to rotate freely for ½ or ¼ turn and the dog<br />teeth on worm gear comes against the dog teeth on sliding clutch with a blow. This is<br />use full when valve is stuck in fully open or closed condition.<br />11. How will you proceed to operate the valve manually after an electrical operation?<br />After electrical operation to operate manually we must tilt the lever provided on the<br />actuator to hand (manual) position by which the sliding clutch gets engaged with<br />hand drive.<br />12. Know the setting procedure for all valves.<br />13. Know the control and power circuit diagram of electrical motorised valve.<br />Question and answers Electrical Maintenance Unit<br />- 86 -<br />Transformers<br />1. What is the colour of silica gel in dry stage and when saturated with moisture in a<br />breather?<br />Dry stage – Deep blue.<br />Saturated stage – Whitish pink.<br />2. What is the static pressure of diaphragm in the transformer explosion vent?<br />5 psi.<br />3. In buchholz relay how the top and bottom mercury switches are connected?<br />Top mercury switch is connected for alarm and bottom switch for trip.<br />4. What is the normal value of moisture content allowed in transformer oil?<br /><10 ppm.<br />5. What is the vector group of distribution transformer?<br />Dy11.<br />6. What are the losses in a power transformer and mention how these losses can be<br />minimised?<br />There are two losses in a transformer.<br />a. Iron losses.<br />b. Copper losses.<br />Iron losses – Iron losses constitutes of two losses.<br />a. Eddy current losses, these are due to the induced emf in the core, which<br />constitutes a current in the core. These will heats up the core.<br />Eddy current losses can be minimised by using laminated core immersed in<br />varnish. This provides a high resistance between the laminations and thus eddy<br />current in reduced.<br />b. Hysterisis losses, these are due to the magnetic reversal of current by which there<br />is friction between molecules of core and heat is generated.<br />Hysterisis losses can be minimised by selecting proper magnetic material, like<br />silicon steel.<br />Copper losses – these losses are due to the resistance of the winding, which is equal<br />to I2rt (calories). These losses are depends on load. That is the losses are<br />increased to the square of the load current<br />I – current through winding.<br />r – resistance of winding.<br />t – time duration.<br />Question and answers Electrical Maintenance Unit<br />- 87 -<br />7. Define the percentage voltage regulation and efficiency of a power transformer.<br />Voltage regulation is the difference between no load voltage and full load voltage by<br />no load voltage.<br />Voltage regulation = No load voltage – Full load voltage / No load voltage.<br />Voltage regulation is mentioned in % (percentage).<br />% Voltage regulation = No load voltage – Full load voltage * 100 / No load voltage.<br />Efficiency of a transformer is the ratio of output in watts and input in watts.<br />Efficiency = Output in watts / Input in watts.<br />% Efficiency = Output in watts * 100 / Input in watts.<br />% Efficiency = Output in watts * 100 / Output in watts + losses.<br />8. Mention the important parts of a power transformer and their purposes.<br />Conservator: This allows for shrinkage and swelling of transformer oil. When the oil<br />is heated up it swells and rises to the conservator. When cools down it goes back to<br />main tank. Conservator reduces sludge formation of oil because only the oil surface<br />in conservator is exposed to atmosphere where oil in the main tank is not exposed to<br />atmosphere.<br />Breather: It provides dry sir to conservator when transformer breathes. That is when<br />there is shrinkage of oil atmospheric air enters conservator through breather. The<br />moisture is absorbed in breather by silica gel.<br />Buchholz relay (gas operated): If there is an initial fault, heating up of core, high<br />resistance joints heating up by conduction through insulation and supports. There is<br />heating up of oil, which breaks down and gases are released. This gas actuates the<br />mechanics in the relay, There by closing contacts of mercury switches for alarm.<br />Also if there is a short circuit, the buchholz relay will trip the transformer. Also if<br />there is any leakage of oil through bushing etc and oil level comes down the relay<br />will give alarm and also will trip the transformer if transformer oil level comes down<br />the point. Gases can be taken from the relay to identify nature of fault.<br />Explosion vent: It provided on transformer main tank, provided with two Bakelite<br />diaphragm which break when the pressure exceeds 5 psi in the transformer tank and<br />relieve the pressure.<br />Core: To provide low reluctance path for the magnetic lines of force. It carries both<br />the HV and LV windings.<br />HV Winding: High voltage is given to HV winding and low voltage is taken from<br />the LV winding.<br />LV Winding: Low voltage is given to LV winding and high voltage is taken from the<br />HV winding.<br />Cooling tubes: These are provided to cool the transformer oil so that the heat of oil<br />will be given to the atmosphere.<br />HT bushing: Carries the HV terminals.<br />LT bushing: Carries the LV terminals.<br />Question and answers Electrical Maintenance Unit<br />- 88 -<br />Tap changer: this is provided so that we can get the required voltage out put. There<br />are two types of tap changer. Online tap changer and off line tap changer.<br />Question and answers Electrical Maintenance Unit<br />- 89 -<br />9. Mention the properties of transformer oil.<br />Properties of transformer oil are,<br />a) Colour – pale yellow.<br />b) Moisture content - <10 ppm.<br />c) Acidity (KOH/gramme of oil) – <0.1.<br />d) Dielectric strength – limit value is 45 kV and preferable value is 60 kV.<br />e) Flash point - 141º C.<br />f) Inter surface tension – 30 to 40 dynes / cm or 0.3 to 0.4 Newton.<br />g) Resistivity –<br />10. How explosion vent works?<br />Explosion vent is provided on the transformer tank to relieve pressure if the pressure<br />in the transformer exceeds 5 psi. It is swan neck shaped having two Bakelite<br />diaphragms. One at top and another at bottom. These break if the static pressure<br />increases to 5 psi. Wire meshes are provided below the bottom diaphragm and above<br />the top diaphragm. When there is any breakage due to excess pressure the bottom<br />wire mesh prevents broken pieces from entering transformer tank and the wire mesh<br />provided above the top diaphragm protects the diaphragm from any external damage.<br />There is an oil level indicator provided above the bottom diaphragm. It indicates the<br />level of oil in the vent if the bottom diaphragm ruptures.<br />A ruptured diaphragm must be immediately replaced. Also we should check the top<br />diaphragm for any external damage.<br />11. Explain the operation of silica gel breather.<br />Silica gel breather is used in a transformer to provide dry atmospheric air to the<br />conservator when transformer breathes. The breather consists of an inner container<br />and outer container. The inner container contains silica gel, which absorbs moisture.<br />An oil bath in provided at the bottom of breather so that the silica gel will not be in<br />direct contact with the atmosphere. Also it will trap dust and dirt entering the<br />breather. Dry silica gel will be deep blue in colour. After it gets saturated with<br />moisture it will turn into white pink. The change of colour silica gel can be viewed<br />externally through transparent viewer provided on the breather. When the silica gel is<br />saturated with moisture it must be replaced or regenerated or recharged. Silica gel is<br />recharged by heating it to a temperature of 250º F to 300º F till the deep blue colour<br />of silica gel is got back.<br />Question and answers Electrical Maintenance Unit<br />- 90 -<br />12. Explain with diagram the operation of on load tap changer.<br />Tap changer (ON LOAD type) can be used to increase or decrease transformer<br />output voltage without break in the voltage to the load.<br />The tap changer consists of a diverter switch. The odd taps are taken on one side and<br />even on another side as shown in figure.<br />The diverter switch is provided so that there will be no break in the supply to the<br />load and also no cut of transition resistance when the tap changing is achieved.<br />Transition resistors are provided to limit the current when the windings are shortcircuited<br />by the diverter switch.<br />Operation: In the above figure, the voltage at tap 2 is 406 V. the position of diverter<br />switch is shown. It short-circuited with transition resistance.<br />We want to increase voltage to 420 V at tap 5. When we begin to change the tap the<br />diverter switch connects 2 transition resistance and when the tap changeover is<br />achieved that is when the tap changer reaches tap 5, the diverter switch short circuits<br />transition resistance and thus the resistance is eliminated. The diverter switch<br />switching time is very high. This is to reduce arcing, which can decompose the oil.<br />The whole assembly is immersed in oil. OLTC is connected to HV side, because in<br />LV side current handled will be more. But in HT side current to be handled is lesser<br />than LV side.<br />Question and answers Electrical Maintenance Unit<br />- 91 -<br />13. Explain the procedure for finding out the vector group of a transformer.<br />Procedure: Take the nameplate details. See from nameplate what group the<br />transformer belongs. Suppose nameplate says that transformer belongs to Dy11.<br />Take the IR value between<br />a) HV and LV with body grounded. That is between A2 – a2, B2 – b2, C2 – c2.<br />b) HV and body with LV grounded and<br />c) LV and body with HV grounded. A2 c2 a2<br />C2 B2 b2<br />Connect A2 to a2 and give low voltage (415 V) to HV side. Measure voltage between<br />a. C2 – b2 􀃆 410 V (example).<br />b. C2 – c2 􀃆 395 V (example).<br />c. B2 – b2 􀃆 395 V (example).<br />d. B2 – c2 􀃆 395 V (example).<br />Draw the diagram of Dy11 and check that the readings got are correct.<br />A2<br />a2<br />30º<br />b2<br />c2<br />According to the fig. C2 B2<br />C2 c2 = B2 b2 = B2 c2.<br />And C2 b2 will be greater than C2 c2 , B2 b2, B2 c2.<br />That is C2 b2 >> C c2.<br />If these conditions are satisfied then that transformer belongs to vector group Dy11.<br />A2<br />/ a2<br />For Dy1 transformer<br />B2 c2 will be greater than B2 b2, C2 c2, C2 b2.<br />That is B2 b2 >> B b2. c2<br />C2<br />b2 B2<br />For Yy0 transformer A2/a2<br />B2 c2 = C2 b2<br />B b2 = C c2<br />B b2 and C c2 will be lesser than B2 b2 and C2 b2.<br />C2/c2 B2/b2<br />Question and answers Electrical Maintenance Unit<br />- 92 -<br />Turbine Generator Basics<br />1. Explain the principle of working of impulse steam turbine.<br />When steam enters the turbine it suffers a change in direction and momentum, which<br />gives rise to the rotation of the turbine. There will be no drop in pressure in impulse<br />steam turbine.<br />2. Mention the four processes involved in Renkine cycle.<br />Saturated liquid line.<br />4 1<br />Saturated vapour line.<br />3’<br />3 2<br />1 – 2 􀃆 Expansion process.<br />2 – 3 􀃆 Constant pressure heat rejection.<br />3 – 3’􀃆 Reverse adiabatic expansion.<br />3 – 4 and 4 – 1 􀃆 Constant pressure heating.<br />3. What are the methods of removing moisture from turbine?<br />a. External method by moisture separator and reheater, which separates the moisture<br />and reheates the steam.<br />b. Internally by stainless steel mesh, which reduces moisture (water particles) to<br />1%.<br />c. By main steam reheat.<br />4. Define capacity factor.<br />Capacity factor can be defined as net power produced by the plant divided by perfect<br />net power that can be produced in the plant.<br />Capacity factor = Net power produced / Perfect net power produced.<br />5. What is the purpose of turbine governing system?<br />Turbine governing system governs the speed of the turbine with the help of<br />centrifugal governer. It reduces the steam inlet when turbine over speeds.<br />6. What are the benefits of feed water heating?<br />a. It improves the plant efficiency.<br />b. Feed water is heated nearer to saturation point thus thermal shock to boiler is<br />avoided.<br />Question and answers Electrical Maintenance Unit<br />- 93 -<br />7. Mention parameters monitored by turbovisory instruments.<br />a. Speed governing.<br />b. Eccentricity monitoring.<br />c. Vibration monitoring.<br />d. Valve position monitoring.<br />e. Temperature monitoring.<br />f. Pressure monitoring.<br />g. Level monitoring.<br />h. Gas leak monitoring.<br />i. Conductivity monitoring.<br />j. Flow monitoring.<br />8. What are the purposes of turning gear?<br />Turning gear is used to run the turbine from rest to low speed and from normal<br />running speed to low speed with the help of barring motor to eliminate the hogging<br />and sagging of turbine because of the high temperature.<br />9. Explain the differences between the two types of feed water heaters.<br />a. Open type: In which bleed steam and condensed water are mixes directly and there<br />is also dearation of steam.<br />b. Closed type (shell type): It has tubes and shell. The water passes through the tubes<br />and steam passes through shell. The heat exchange takes placcce through the<br />metal tubes.<br />10. Why condenser back-pressure must be low? How it is achieved?<br />Condenser back- pressure must be low, because steam should be dumped into the<br />condenser so as to recycle it to boiler through the recycle process. It improves<br />efficiency of the turbine, as the heat rejection is less. It is achieved by the help of<br />ejectors and also passing cold water in the condenser through the tubes of the<br />condenser so that maximum vacuum can be obtained.<br />11. What are the materials used for TG rotor and blades?<br />TG rotor is made up of alloy steel and blades are made up of stainless steel.<br />12. Define the term heat rate?<br />Heat rate is defined as the heat supplied in to the plant in Btu by power generated or<br />output by the plant in kWh.<br />Heat rate = Heat supplied in Btu / Power output in kWh.<br />13. What is the purpose of gland steam system?<br />Gland steam system is provided to arrest the steam leak from the turbine and to<br />protect the air ingress into the turbine.<br />Question and answers Electrical Maintenance Unit<br />- 94 -<br />14. Explain the main difference between impulse and reaction turbine.<br />When the inlet pressure of steam to the turbine is equal to outlet pressure of steam<br />from the turbine the turbine is called the impulse turbine. In this type the heat is<br />added is very less.<br />In reaction turbine the outlet pressure of steam is less than the inlet pressure of the<br />steam. There is reduction in pressure with the increase in kinetic energy.<br />15. What is meant by hydrodynamic film lubrication?<br />In high-speed turbines the lubricating oil will be at the sides and there is metal to<br />metal contact when turbine at rest. When the turbine speeds up there is pressure<br />pushing the oil through the metal to metal contact. When turbine finally achieves its<br />speed the oil film breaks the barrier and it takes the load on itself. This is<br />hydrodynamic lubrication.<br />16. How does the hydrostatic lubrication differ from hydrodynamic type?<br />In hydrostatic lubrication which is used in slow speed turbines the lubricating oil is<br />pressurised externally where as in hydrodynamic system it forms oil film by its speed<br />which pushes the lubrication oil to form film.<br />17. What are the functions of dearator?<br />Dearator removes non-condensable gases (O2), which gets added in the steam and it,<br />mixes steam with the condensed water for feed water heating. This is a contact type<br />feed water heater.<br />18. Why non-return valves are provided in the steam extraction lines?<br />Non-return valves are provided because when the turbine trips there will be an<br />instant drop in pressure inside the turbine. But there will be steam in feed water<br />heaters, which is at high pressure. These will rush in to the turbine and overspeed<br />will be there in turbine. So non-return valves are provided in steam extraction lines<br />to prevent over speeding of turbine.<br />19. What is the function of the steam traps?<br />During startup the steam traps will bypass turbine drains.<br />20. What do the term sensible heat and latent heat mean?<br />Sensible heat: We can measure the rise in temperature. When we add more heat to a<br />substance. Example – heat that added to water from 0º C to 100º C. This added heat<br />is measured as sensible heat.<br />Latent heat: Though there is addition of heat there will be no rise in temperature.<br />This is latent heat. Example – when water boils at 100º C though we added more<br />heat the temperature remains at 100º C till all water becomes steam.<br />Question and answers Electrical Maintenance Unit<br />- 95 -<br />21. How are the generator rotor and stator cooled?<br />Passing highly DM water through the hollow conductor of the generator cools<br />generator stator and rotor is cooled by hydrogen.<br />Question and answers Electrical Maintenance Unit<br />- 96 -<br />22. What is function of seal oil system?<br />Seal oil prevents the leakage of hydrogen from the generator casing to the<br />atmosphere, where it can form explosive mixture. Seal oil is at higher pressure than<br />hydrogen.<br />23. What are the base load and peak load power stations?<br />During certain periods the load demands are very high. Example the morning when<br />all factories operate. During these time certain power plants like thermal plants gives<br />this extra power required. These are the Peak load stations, which operates at certain<br />periods.<br />But during the rest of period that is when there is no peak power demand there are<br />some power stations, which cater to the base load always runs giving power to the<br />grid. These stations are producing power at constant rate. These stations can not be<br />easily stopped or restated. Nuclear power station comes under Base load power<br />station category.<br />24. What are two types of governing system.<br />Throttle governing system: In this a valve (just like tap water controlling) which<br />reduces the steam pressure controls the steam flow. This has very less efficiency.<br />Nozzle governing: In this the steam floe is reduced but the pressure remains the<br />same. This is achieved by four valves in which when one is closed to 25% of steam<br />is reduced. This is efficient way of governing.<br />Question and answers Electrical Maintenance Unit<br />- 97 -<br />Conventional system<br />1. How turbine oil purification is achieved?<br />Oil purification is achieved by centrifugal operation.<br />2. Why morpholine is used?<br />Morpholine is used for pH control.<br />3. By which material condenser tubes are made?<br />Condenser tubes are made up of aluminium brass.<br />4. How boiler level is controlled?<br />Feed water control valves controls boiler level.<br />5. Where magnetic filter is used?<br />Magnetic filter is used in stator water lines to remove magnetic particles from the<br />DM water.<br />6. What is the purpose of accelerator governer?<br />The purpose of accelerator governer is to cut of steam momentarily when large<br />electrical loads are taken to prevent turbine speeding up.<br />7. What is the use of jacking oil pump?<br />Jacking oil pump is used to initial lifting of turbine rotor by hydrostatic lubrication.<br />8. What is the use of supplementary oil tank?<br />Supplementary oil tank is used to collect the oil drains from the CIES valves.<br />9. How dearator pressure is maintained?<br />Dearator pressure is normally maintained by extraction steam.<br />10. How dearator pressure is maintained after turbine trip?<br />Pegging steam is used to maintain dearator pressure after turbine trip.<br />11. Where trust bearing is provided?<br />Trust bearing is provided between HP turbine and LP turbine rotor.<br />12. How dissolved oxygen control is achieved in feed water system?<br />Hydrazine is added to feed water system to control dissolved oxygen.<br />13. How seal oil pressure is maintained?<br />Differential pressure regulator maintains the seal oil pressure at 0.7 kg / cm2 higher<br />than H2 pressure.<br />Question and answers Electrical Maintenance Unit<br />- 98 -<br />14. What is used to purge hydrogen from the generator casing?<br />During generator purging CO2 is used to purge out H2 from the casing.<br />Question and answers Electrical Maintenance Unit<br />- 99 -<br />15. Why class B trip is provided?<br />Class B trip is provided to prevent damage and over speeding of the turbine.<br />16. How lub oil supply is maintained?<br />Lubrication oil is supply is maintained by outlet oil from turbine oil pumps during<br />normal operation and jacking oil pump during startup.<br />17. What is the function of speeder gear?<br />Speed raising beyond governor takeover speed upto full speed is achieved by raising<br />and lowering the speeder gear.<br />18. How gland-sealing steam is supplied?<br />Gland sealing speed is supplied from main steam line.<br />19. Why exhaust sprays are provided?<br />Over heating of last stage LP blades is avoided by exhaust sprays by CEP.<br />20. What is the use of vacuum breaker?<br />In case of loss of seal oil to generator seals vacuum breaker is used to bring TG to<br />rest very quickly.<br />21. How relay oil is supplied?<br />Relay oil is supplied from the main oil pump for the operation of governing system.<br />Question and answers Electrical Maintenance Unit<br />- 100 -<br />Electrical system<br />1. What are the main two divisions of MAPS electrical system and what do you<br />understand by it?<br />The two main divisions of MAPS electrical system are<br />a. Main output system. Output system supplies power to the grid. Generated voltage<br />is stepped up to 220 kV from 16.5 kV and supplied to grid.<br />b. Station service system. This system supplies the load inside the power station.<br />The generated voltage is stepped down to 6.6 kV and 415 V from 16.5 kV and<br />supplies to auxiliary loads.<br />2. List out the components of station output system.<br />Main generator, Generator transformer, PT, CT, CVT, lightning arrestor, wave trap,<br />main 220 kV bus, transfer bus, SF6 circuit breakers and isolators, line protection<br />scheme, GT and Generator protection scheme, bus bar protection scheme etc.<br />3. Why earth switches are provided in 220 kV bays?<br />When bay CB trips, both end (station and grid) CB will trip. The earth switches are<br />provided because the grid will always be alive so to prevent any shocks to the<br />operator or maintenance personnel who is working on the line or bay due to<br />accidental energizing of the bus.<br />4. What is the purpose of CVT (capacitance voltage transformer)?<br />Purposes of CVT are<br />a. To indicate if line is charged or not.<br />b. To synchronize grid with generator.<br />c. For power line communication and carrier tripping.<br />5. What are the protections provided for 220 kV lines and bus bars?<br />a. Bus bar differential protection.<br />b. Distance protection.<br />c. Over current protection.<br />d. Earth fault protection.<br />6. What are the main sources of power supply to 6.6 kV buses?<br />a. Unit transformer which steps down the generated voltage to 6.6 kV from the<br />generator.<br />b. Start up transformer, which steps down the grid voltage to 6.6 kV.<br />7. List some important loads to 6.6 kV buses.<br />a. Auxiliary transformers.<br />b. PHT motors.<br />c. BFP motors.<br />Question and answers Electrical Maintenance Unit<br />- 101 -<br />d. CEP motors.<br />e. CCW motors.<br />f. Chiller motors.<br />g. Pressuring pump motors.<br />Question and answers Electrical Maintenance Unit<br />- 102 -<br />8. What type breakers are provided in 6.6 kV buses?<br />ABB. Make, SF6 gas, 1250A and 2000A capacity circuit breakers are provided in<br />6.6 kV buses.<br />9. What is the difference in action of lock out pressure signal on 6.6 kV and 220 kV<br />breakers?<br />When lock out signal comes to 6.6 kV breakers the breaker will trip. Where as in<br />case of 220 kV breakers the breaker will not trip. If the breaker is open it will be<br />open only and can not be closed. If it is in closed condition it will be closed.<br />10. For how long 220 V DC batteries can supply power UPS?<br />220 V DC batteries can supply Power UPS for 30 minutes. Within this time class III<br />power supply should be restored by DG’s.<br />11. What do you understand by station black out?<br />When class IV and class III power supply fails and DG’s cannot be started and also<br />this condition prevails for 5 minutes then it is called station black out condition.<br />12. What are the sources of power supply to class I bus?<br />a. Through control UPS 240 V AC.<br />b. Through control UPS 220 V DC backed by 220 V batteries.<br />c. Through control UPS 48 V DC backed by 48 V batteries.<br />13. What are the lighting systems adopted in KGS?<br />There are two systems.<br />a. Normal lighting with class IV power supply.<br />b. Emergency lighting with class II power supplies and in control room with class I<br />power supplies.<br />Question and answers Electrical Maintenance Unit<br />- 103 -<br />Measuring instruments<br />1. What are the two main classifications of analog instruments?<br />The two main classifications of instruments are,<br />a. Absolute instruments. Example tangent galvanometer.<br />b. Secondary instruments. Example ammeter, voltmeter. Analog instruments are<br />classified according to their electrical quantity they measure. Example frequency<br />meter, voltmeter, etc. Principles they work are moving coil, induction.<br />2. What are three types of secondary instruments?<br />The three types of secondary instruments are,<br />a. Indicating type: It only indicates the electrical quantity measured. Example:<br />Ammeter, Voltmeter, Frequency meter etc.<br />b. Integrating type: It integrates (sums up) the quantity being measured. Example:<br />Energy meter.<br />c. Recording meter: It records as well as indicates the electrical quantity being<br />measured. Example: 3 pen graphical recorder.<br />3. Give three operating forces acting on indicating instruments.<br />a. Deflecting force.<br />b. Controlling force.<br />c. Damping force.<br />4. What are the advantages of digital instruments over analog instruments?<br />a. Human errors are avoided (comparative error) because the output is displayed in<br />form of numbers.<br />b. Power consumption of digital meters are low as compared to analog meters.<br />5. What is the range of resistances that can be measured using following.<br />a. Wheatstone bridge – 1 milli Ω to 11 MΩ.<br />b. Kelvins double bridge – 0.2 micro Ω to 11 Ω.<br />c. Megger – Insulation resistances more than 100 kΩ<br />6. What do you understand by tan delta for a insulating material?<br />Tan delta measurement is done to find the qualities of insulating material. Tan delta<br />is angle between current due to surface leakage or current due to capacitance and the<br />capacitive current. That is Tan δ = Ir / Ic.<br />7. For what purposes transformer ratio meter can be used?<br />Transformer ratio meter can be used for,<br />a. To find the ratio of a transformer.<br />b. To find the phase angle deviation of primary and secondary voltage of<br />transformer.<br />Question and answers Electrical Maintenance Unit<br />- 104 -<br />c. To find the magnitude of magnetizing currents.<br />Question and answers Electrical Maintenance Unit<br />- 105 -<br />8. Draw connection diagram of ammeter, voltmeter, energy meter, and wattmeter.<br />9. Illustrate how can you use a single-phase wattmeter to measure 3 phase reactive<br />power in a circuit?<br />We can measure reactive power of 3 phase circuit by dingle phase wattmeter by<br />connecting the current coil in series with a line or load and connecting the pressure<br />coil across the other two lines.<br />Reactive power = 3√ V * I * sinϕ Watts.<br />10. Draw the basic block diagram of digital meter and explain the function of each<br />block.<br />Alternator A/D converter BCD counter decoder &amp; LCD display<br />Vx<br />Alternator: It reduces the unknown voltage to a small value. Because the reference<br />voltage is very less and the unknown voltage is maximum.<br />A/D converter: It converts the analog signal from alternator to digital signals.<br />BCD counter: It counts the number of pulses (binary counter).<br />Decoder and display: It decodes the binary code to decimal form and gives a visual<br />display of it.<br />11. Draw a neat sketch and explain the use of CT and PT for measurement of power in a<br />single-phase circuit?<br />If wattmeter of proper range is not available or if voltage and current ranges are high<br />we can usr CT and PT of suitable ratio. Connect the CT and PT as shown in figure.<br />The reading of wattmeter can be multiplied by the ratio to get the actual power.<br />CT PT<br />CC<br />WATT METER<br />PC<br />Question and answers Electrical Maintenance Unit<br />- 106 -<br />12. Explain the construction and working of moving coil instrument.<br />Construction: The moving part is a coil wound on a light aluminium frame,<br />mounted on a shaft made of stainless steel which is pivoted at either ends on jewel<br />bearing made of sapphire. The coil is placed between poles of a permanent magnet.<br />Moving system is made light as far as possible to have high torque and weight ratio.<br />There are two phosphor bronze springs of very less resistance. This is used as<br />terminals for the current to pass through the coil and out of coil. It is also serves the<br />purpose of control force.<br />Aluminium coil former acts as a damping device by eddy current damping. There is<br />a knife edge pointer and a counter weight to avoid its sagging.<br />Working: When ever a current carrying conductor is placed in a magnetic field a<br />force is experiences by the conductor. Moving coil meters work on this principle.<br />Force = BINL<br />As the BNL is constant, force (F) is directly proportional to current (I).<br />That is FαI. Where B is magnetic flux, N is number of turns and L is length of coil.<br />13. Explain the procedure for measurement of earth resistance.<br />14. Explain the working principle of wheat stone’s bridge along with equations under<br />balanced condition. B<br />P kG Q<br />I1 I1<br />A I2 G<br />I2 D<br />X S<br />C<br />Wheat stone’s bridge works on kirchoff’s law. It is used to measure medium range<br />resistances. P and Q are fixed standard resistances. S is standard variable resistances.<br />X is the unknown resistance. G is galvanometer, kG is galvanometer switch and kB<br />is battery switch. No current will flow through galvanometer if the potentials across<br />its terminals are equal. So there will be no deflection of galvanometer. This condition<br />is called the balanced condition.<br />This is achieved by varying S and also by varying P/Q ratio.<br />At balanced condition VAB = VAC And VBD = VCD<br />I1<br />*<br />P = I2*X &amp; I1*Q = I2*S<br />Dividing both I1<br />*<br />P = I2*X<br />I1*Q = I2*S<br />Question and answers Electrical Maintenance Unit<br />- 107 -<br />= P/Q = X/S<br />Unknown resistance (X) = P/Q * S Ω<br />15. Explain the working principle of Kelvins double bridge and procedure for the<br />measurement of terminal resistance.<br />Question and answers Electrical Maintenance Unit<br />- 108 -<br />Protective relays and application<br />1. What you mean by accuracy limit factor?<br />The ratio between the accuracy limited primary current to rated primary current is<br />called the accuracy limit factor.<br />2. What is the characteristic of inverse time over current relay?<br />If the fault current increases the time of the operation of the relay will be decreases.<br />3. What are the two errors in instrument transformer?<br />a. Ratio error.<br />b. Phase angle error.<br />4. Where core balance CT is used?<br />Core balance CT is used in earth fault protection.<br />5. Define knee point voltage of a CT.<br />When the primary of a CT is open circuited and supply (variable) of system<br />frequency is given to secondary, then a 10% increase in voltage constitutes 50%<br />increase in current. That voltage is the knee point voltage.<br />At this point the core is saturated and a little increase in voltage constitutes a great<br />increase in current. kpv decides the opening range of the CT. Above kpv the ratio of<br />transformer will not be applicable.<br />kpv = RCT + RLEADS + RRELAY<br />6. What do you mean by the term 5P10?<br />This indicates the type of relay, Its % error and accuracy limit factor.<br />5 – composite error (Phase angle error + ratio error) 5%.<br />P – Protection CT.<br />10 – Accuracy limit factor.<br />7. Mention the important properties of relay contacts.<br />a. Should be robust in construction.<br />b. Self-cleaning (oxides easily breakdown).<br />c. Corrosion resistant.<br />d. Bounces free and striction free (low contact resistance).<br />e. Able to carry rated continuous current and short time rated current.<br />8. What is a composite error and write down the formula for composite error?<br />Basically composite error = Ratio error + Phase angle error. It is the ratio error<br />integrated over one cycle at steady state of operation.<br />Composite error =100 * 1 oςT (kn * Is – Ip)2 dt<br />Question and answers Electrical Maintenance Unit<br />- 109 -<br />T Ip<br />Question and answers Electrical Maintenance Unit<br />- 110 -<br />9. Define pickup value and reset value.<br />Pickup value: It is the smallest value of actuating quantity when its value is increased<br />from zero to pickup value, the relay will energise.<br />Drop out value: It is the largest value of the actuating quantity when its value is<br />decreased from pickup value, the relay will reset or de-energize.<br />10. Draw the circuit diagram for finding out the knee point voltage and explain the<br />procedure.<br />0 – 5 A<br />A CT<br />V 0 – 300V Sec Primary<br />240 V AC<br />Variac V Saturation<br />kpv = RCT + RLEADS + RRELAY<br />Knee point<br />Ankle point A<br />Connect the circuit as shown. O/P of variac should be zero. Increase it to 5 Volts and<br />take down the value of current from the ammeter. Now increase the voltage by 10%<br />(5 + 10% = 5.5 V) and take the current reading. Now increase the voltage by 10%<br />(5.5 V +0.55 V =6.05 V) and note down the current. Now keep on increasing voltage<br />by 10% and note down current reading. At some value there will be 50% increase in<br />current for 10% increase in voltage.<br />Example 40 V􀃆 0.2 A<br />40.4 V􀃆 0.3 A (0.2 + 50% = 0.3 A).<br />That point is the knee point voltage of that particular CT. From this point onwards a<br />little increase in voltage will lead to a large increase in current, because the core is<br />saturated fully. When we plot all the values on a graph taking current as X-axis and<br />voltage as Y-axis, we will get the above graph. Protective relays operate between<br />ankle point and knee point. Above this they cannot detect the fault correctly.<br />Measuring CT operate in the ankle region.<br />Question and answers Electrical Maintenance Unit<br />- 111 -<br />11. Explain the procedure for finding out the polarity and ratio test in a CT with circuit<br />diagram.<br />Polarity test: Connect the circuit as shown in figure with a battery, switch and<br />AVOmeter. Now momentarily close the switch S and see the deflection in the<br />AVOmeter. If it is in the direction as shown in the figure, then the polarity of the CT<br />is correct. If it is in opposite direction the polarity of CT is not correct. Polarity test is<br />very important because if polarity is not correct in differential protection the relay<br />will fail to act when fault occurs.<br />Ratio test: Connect circuit as shown in figure (2). Slowly increase the current. Take<br />down the readings of A1 and A2. Then see whether it confirms to reading of<br />nameplate. Ratio = A1/A2.<br />SECONDARY INJECTION KIT<br />+AVO - A A1<br />0 – 30 A<br />+ - 240 V AC<br />S1 S2<br />P1 P2<br />+ -<br />S B<br />POLARITY TEST (FIG 1) Fig – 2 Ratio test<br />Ratio – A1 : A2 A2<br />A 0 – 15A<br />12. Explain the principle of operation of attracted armature relay with equation and<br />characteristics curve.<br />Principle: It works on the principle that when a current is passed through a coil<br />magnetic lines of force develop and the coil behaves like a magnet. When we place a<br />magnetic material inside the coil it is attracted.<br />In attracted armature type of relays there is a spring that keeps the contact open, a<br />plunger that tends to close the contact and a coil through which current is passed.<br />The spring force and magnetic force oppose each other. When these both are equal<br />the relay will pickups.<br />At verge (time) of pickup Instantaneous select<br />k1 I2 = k2<br />f = k1 I2 = k2 Time Time delay select<br />I = k2 / k1<br />Where f – force.<br />k1 – magnetic force constant.<br />k2 – spring tension constant. current<br />I – current in the coil.<br />Question and answers Electrical Maintenance Unit<br />- 112 -<br />We can see the inverse characteristics from the above formulae. Usually attracted<br />armature relays are instantaneous. That is there is no intentional (fixed) time delay. If<br />we want a time delay we can add a slug in the armature core.<br />Question and answers Electrical Maintenance Unit<br />- 113 -<br />13. Mention the initial commissioning checks on CT’s, PT’s and relays.<br />Commissioning checks on<br />CT<br />a) Terminal marking correctness.<br />b) Polarity of terminals.<br />c) Insulation resistance between primary and secondary.<br />d) Insulation resistance between primary to earth and secondary to earth.<br />e) Magnetization characteristic and knee point voltage test.<br />f) Ratio test.<br />PT<br />a) Terminal markings.<br />b) Polarity checks of terminals.<br />c) Insulation resistance between primary and secondary.<br />d) Insulation resistance between primary to earth and secondary to earth.<br />e) Ratio test.<br />f) Whether PT can supply as per the burden of load check.<br />RELAYS<br />a) Pickup and dropout value check.<br />b) Insulation resistance of contacts and relay coil.<br />c) Time delay (if relay is not instantaneous), operating time value check of relay.<br />d) See that the correct circuit breaker trips on energisation of the particular relay.<br />e) Continuity checks of contacts after energisation of relay.<br />f) See if plug-shorting contacts are correct.<br />g) See if CT’s and PT’s are corrected in correct polarity.<br />h) Burden check of relay.<br />i) Primary injection test.<br />j) Secondary injection test.<br />Question and answers Electrical Maintenance Unit<br />- 114 -<br />14. Explain with simple diagram the core balance CT.<br />In core balance CT all the three phases go through the core and the resultant<br />magnetic flux is zero. Because the flux of three phases cancel each other. So the<br />secondary output of CT is zero and the relay will not energise.<br />When there is a earth fault in one of the phase the fluxes cannot balance each other<br />and there is a voltage induced in secondary of the CT and the relay is energised to<br />trip the circuit. Saturation is no problem because the core size is very big.<br />+R<br />Ground fault<br />R R Y<br />Relay dropped Relay pickup<br />B R<br />R Y B R Y B - R<br />Normal operation. During earth fault. Resultant diagram.<br />Question and answers Electrical Maintenance Unit<br />- 115 -<br />Logics and circuits<br />1. Give the definition of following.<br />27C Closing circuit supervisory relay<br />27T Tripping circuit supervisory relay<br />3C Interposing relay (closing coil)<br />3T Tripping coil<br />52 AC circuit breaker<br />88 Auxillary motor (spring charging motor)<br />52Y Anti-pumping relay<br />86.1 Lockout relay<br />42 Main contactor<br />50 Instantaneous over current relay<br />50N Earth fault relay<br />94 Trip or Trip free relay<br />49 Thermal overload relay<br />49S Stalling protection relay<br />27 Supervisory relay<br />64 Ground protection relay<br />2. What is the operating voltage of 3C?<br />48V DC.<br />3. DC relay coil or contactor coils must be connected to which side?<br />Negative side of the DC supply to avoid galvanic effect on the coil, which will corrode the coil.<br />4. How special current limiting resistance is connected with the seal in contact?<br />Special current limiting resistance is connected in series with the seal in contact.<br />5. How you will connect start and stop push button to control the motor from two different places?<br />Start push button should be connected in parallel and stop push button in series in the circuit.<br />6. What are the basic principles of ED?<br />Basic principles of ED are,<br />a) All the contacts of corresponding relays and contactors are shown in de-energised condition.<br />b) Control circuit gives us idea about ON / OFF selection of motor, fuse rating, forward reverse<br />control, seal in protections etc.<br />c) Power circuits are drawn in thick lines and control circuits are drawn in thin lines.<br />d) When relay or contactor energises normally open contact closes and normally closed contact<br />opens.<br />e) Auxillary contacts acts with main device such as contactors and relay.<br />7. What is anti pumping?<br />When a breaker is closed on fault condition there will be continuous tripping and closing of the<br />breaker because 3C is energized. Anti pumping in circuit avoids frequent tripping and closing of<br />circuit breaker when the breaker is closed in fault condition.<br />Question and answers Electrical Maintenance Unit<br />- 116 -<br />Switchgear<br />1. Mention the commissioning tests on breaker and bus bars.<br />Breaker<br />a. Milli volt drops test between the interrupting contacts and between the isolator<br />contacts.<br />b. Closing and opening timing of the breaker for 5 times.<br />c. Checking whether the breaker trips or closes when the logics are fulfilled.<br />Bus bars<br />a. Milli volts drop test for the contact resistance value.<br />b. Tightness of the joints.<br />c. IR values between phase to phase and phase to ground.<br />2. Explain clearly the three positions in 415 V breaker.<br />a. Service position: Power connections and control connections are available to the<br />breaker.<br />b. Test position: Power connections are cut off but control connections are available<br />to the breaker and it can be tested.<br />c. Disconnect position or rack out position: This is for maintenance of the breaker<br />and in this positions both the control and power connections are not available.<br />3. What do mean by trip free system in breaker?<br />In trip free the breaker is free to trip at any time. If both close and trip signal is<br />present at same time (instant) the breaker will attempt to close and positively trip.<br />When the breaker trips it will not close again even if closing signal exists because of<br />anti pumping feature.<br />4. What is the purpose of spring charging in 415 V breaker?<br />If the breaker is to be closed and tripped manually the closing time and tripping time<br />would vary from person to person. Also it would not be very fast. So spring charging<br />is provided. It gives uniform timings irrespective of the operator and its action is fast<br />and closing and tripping time is very less.<br />Question and answers Electrical Maintenance Unit<br />- 117 -<br />5. What are the built in protections provided in 415 V breaker?<br />a. DINF (making current magnetic release)<br />This consists of a laminated magnetic circuit. This is placed under the lower<br />current terminal pole. This is provided for all the three poles. It has a core that<br />rotates in the air gap. It is held by spring. During protection the magnetic forces<br />developed overcome the spring tension and the core is attracted. The mechanical<br />force developed is used to trip the breaker. This protection acts during the closing<br />of breaker if any fault exists. The current is set to 5 times the rated current.<br />b. DIRS (short time magnetic release)<br />The construction is same as DINF, but it has a mechanical timer, which can be set<br />accordingly. This protection acts when any fault comes during breaker in service.<br />The current rating is set to 3 to 8 times the rated current.<br />c. DIT – S (thermal over load protection)<br />This consists of a three bimetallic strip, which gets heated up when over loaded<br />and trips the breaker by a lever. It is placed in front of the breaker. Setting range<br />is 60% to 100%.<br />Question and answers Electrical Maintenance Unit<br />- 118 -<br />MCC<br />1. What are the advantages of MCC?<br />a. Starters and contactors all will be a standard size.<br />b. Spares inventory will be less.<br />c. Cost of cables will be less because same size of cable is used for all capacity and<br />gland holes can be provided before hand.<br />d. Maintenance and trouble shooting is easier in MCC.<br />2. What type of motor starting adopted in MCC?<br />DOL (direct on line) starter.<br />3. What is the purpose of grounding secondary of the control transformer?<br />To protect the operating personnel from high induced voltage.<br />4. Based on what factors will you select rating of components for a starter cell?<br />Factors for selecting rating of components are,<br />a. Capacity of load.<br />b. Type of starting.<br />c. Duty (continuous or intermittent).<br />d. Type of protection.<br />e. Nature of starting (acceleration time is slow or fast).<br />5. What maintenance checks you will do for an MCC cells and MCC panel?<br />Maintenance checks on MCC cells<br />a. Ensure that the load is tripped from control room and switch is in off position.<br />Switch off the isolator at MCC cell.<br />b. Open the door and rack out the cell into isolation position.<br />c. Check the tightness of terminal of contactor, 3C, control transformer, control<br />fuses, wipe in contacts, power cables, etc.<br />d. Check the tightness of component mounted.<br />e. Look for any charred components or terminals.<br />f. Check the IR value of 3C, contactor, control transformer, isolator etc.<br />g. Check the isolator double switch feature.<br />h. Check the OLR and calibrate the OLR.<br />i. Check the pick and drop out value of contactor, 3C.<br />j. Check the fuses for healthiness and fuse carriers for proper contacts.<br />k. Clean the arc-chutes of the contactor and clean all the components of the MCC<br />cell properly.<br />l. Check the resistances of control transformer, contactor, 3C etc.<br />m. Check the tightness of control cable at main TB compartment.<br />Question and answers Electrical Maintenance Unit<br />- 119 -<br />Maintenance on MCC panel.<br />a. Ensure the permit and all isolations.<br />b. Open the bus bar chamber and discharge the bus bar.<br />c. Disconnect the cables connected to bus bar and take the IR value of bus bar and<br />cable individually. Connect it properly and tight it to proper torque.<br />d. Check the tightness of nut and bolts and cables connected to buses.<br />e. Open the main TB compartment and check the tightness of all cables and clean it.<br />f. See the tightness of power terminal compartment and clean it.<br />g. Check that cables are supported properly.<br />h. Do checks on CT, PT used for indication purposes.<br />i. Clean the entire MCC panel properly and take the IR value.<br />j. Carry the checks on relays, which are used in the MCC panel.<br />k. See for proper earthing connection and tightness of the earthing connections.<br />l. See for proper house keeping.<br />6. What is the difference between an auto reset and manual reset overload relay?<br />a. Auto reset relay closes its contacts when the bimetallic strip gets cooled. In<br />manual reset relay we have to manually reset the relay because even though<br />bimetallic strip cools its contacts are not closing without manual reset. L &amp; T type<br />OLR have only manual reset and siemens type has both manual and auto facility.<br />b. Auto reset over load relay is reset by switching OFF the respective had switch<br />and again switching it ON.<br />c. Manual over load relay is reseted by pushing the reset button provided on the<br />MCC cell.<br />Question and answers Electrical Maintenance Unit<br />- 120 -<br />Batteries and charger<br />1. How you will prepare electrolyte for a lead acid battery?<br />While preparing electrolyte for lead acid battery sulphuric acid is added to distilled<br />water.<br />2. How battery capacity is expressed?<br />Always battery capacity is expressed in Ampere – hour.<br />3. What is the instrument used to measure the specific gravity?<br />The instrument used to measure the specific gravity is called Hydrometer.<br />4. What you mean by SCR?<br />SCR is meant for silicon controlled rectifier.<br />5. Define specific gravity and mention the specific gravity of a fully charged lead acid<br />battery?<br />Specific gravity of a substance is the comparison of density of the substance with the<br />density of pure water.<br />Specific gravity = Density of the substance / density of pure water.<br />= kg / cm2<br />kg / cm2<br />= (No unit)<br />Specific gravity is only number. It has no unit.<br />Specific gravity of pure water is one.<br />Specific gravity of fully charged lead acid battery is 1.215. Specific gravity should<br />always be corrected to 27°C.<br />Corrected specific gravity is equal to indicated specific gravity ± (t - 27°C)*0.0007.<br />Indicated specific gravity = 1.205 and ‘t’ means electrolyte temperature.<br />6. What are the parts of the battery?<br />Parts of the battery are<br />a. Battery container.<br />b. Battery cover.<br />c. Positive plate (Pb o2).<br />d. Negative plate (Pb).<br />e. Cell connector.<br />f. Grid.<br />g. Cell separator (porous material).<br />h. Sediment chamber.<br />i. Positive and negative terminals.<br />j. Vent plugs.<br />Question and answers Electrical Maintenance Unit<br />- 121 -<br />k. Dilutes sulphuric acid (electrolyte).<br />Question and answers Electrical Maintenance Unit<br />- 122 -<br />7. What are the indications of a fully charged cell?<br />a. The colour of the + ve plates will be dark brown. This can be seen only if the<br />battery has transparent cover.<br />b. Voltage per cell will be a 2.15 volts.<br />c. Gassing in the will electrolyte will indicate. But the current is splitting up water to<br />H2 and O2. Because the positive and negative plates are fully converted to their<br />original constituents.<br />8. What are the difference between primary cell and secondary cell?<br />Primary cell: The electrolyte in primary cell is chemically irreversible. That is once<br />the cell is discharged it cannot be recharged. It should be replaced with a new cell.<br />The cells can supply only low currents and have low efficiency. They supply<br />intermittent current. Their internal resistance is more. These cells are comparatively<br />cheap.<br />Secondary cell: These cells are chemically reversible. They can be discharged and<br />charged. They can supply large currents because their internal resistance is less.<br />These have high efficiency compare to primary cells. These can supply constant<br />current. These are comparatively costly.<br />9. What do you mean by sulphation? And what are the effects of sulphation?<br />Sulphation: During normal discharge of battery Pb so4 is formed. This Pb so4 is<br />chemically reversible by passing current. These split up to their original constituents.<br />But under certain condition crystalline lead sulphate is formed (Example: under<br />charging after some time without trickle charging). This Pb so4 is chemically<br />irreversible. So if the sulphation occurs the battery life decreases. Efficiency<br />decreases and the active material starts falling of the grid.<br />10. Write down the equation for Nickel cadmium battery.<br />Equation for Nickel cadmium battery.<br />Ni (OH4) + Cd +2 kOH 􀃆Ni (OH2) + Cd OH2 + kOH (during charging)<br />(Nickel hydrate + cadmium + potassium hydroxide 􀃆 Nickel hydroxide + cadmium<br />hydroxide + potassium hydroxide.)<br />Ni (OH2) + Cd OH2 + kOH 􀃆 Ni (OH4) + Cd +2 kOH (during discharge)<br />We see that there is no change in electrolyte. It just acts as a catalyst. So there is no<br />need to change the electrolyte.<br />Question and answers Electrical Maintenance Unit<br />- 123 -<br />11. Write down the theory of lead acid battery.<br />A simple lead acid battery consists of positive and negative electrodes Immersed in<br />an electrolyte. The positive electrodes are Pbo2 (lead oxide) and the negative<br />electrodes are Pb (spongy lead). The electrolyte is dilute sulphuric acid.<br />On discharging the battery<br />Pbo2 + Pb +2 H2 so4 􀃆Pbso4 + Pbso4 + 2 H2o<br />Lead acid is converted into lead sulphate. Spongy lead is also converted into lead<br />sulphate and H2 so4 used up in the process. Only water is remain. So the specific<br />gravity of the cell comes down.<br />On charging the battery<br />Pbso4 + Pbso4 + 2 H2o 􀃆 Pbo2 + Pb +2 H2 so4<br />Here the products are converted to their original constituents and the acid is released.<br />So the specific gravity rises as the cell is charged. How much ever larger be the cell<br />the voltage of each cell will be approx. 2.15 V when fully charged.<br />The positive plate is made of a paste lead oxide, lead sulphate that is fitted in a mesh<br />like material and is connected to grid. All positive plates are made common and<br />connected to a grid.<br />The negative plate is made of spongy lead also it is in mesh and connected to grid.<br />These are also grouped together.<br />These plates are separated by a micro porous separator for the diffusion of<br />electrolyte.<br />The electrolyte is prepared by adding sulphuric acid to distilled water drops by drop<br />and stirring it until the reasoned specific gravity is attained.<br />Every thing is placed in a container of hard rubber. The cells of a battery are<br />connected by a cell connector. The container is leak proof.<br />12. What are the parts of a 48V DC charger?<br />Main transformer: This steps down the 3Φ 415V supply to the require value of<br />voltage.<br />Synchronizing transformer for phase sequence: This gives the synchronizing signal<br />to the firing card. That is, the pulses from this card if fed to the firing card. The firing<br />card gives pulses to the SCR of R or Y or B depending upon which phase is positive<br />maximum.<br />Half control module: This has a diode and a SCR for each phase. The firing card<br />controls the firing angle of SCR.<br />Firing card: This gives the firing pulses to SCR depending on phase sequence and<br />the feed back from output.<br />Controller card: This card monitors the output and gives signal to firing card to<br />conduct at certain angle to maintain constant output voltage.<br />Power supply card: This gives power supply for the controller card, firing card and<br />protection.<br />Question and answers Electrical Maintenance Unit<br />- 124 -<br />13. Explain the operation of 48 V battery charger.<br />The supply for the charger is from MCC. The supply is tapped for power supply to<br />control card, PF correction capacitor. LC filter is used for suppressing surge voltage.<br />The main supply is stepped down and given to the half control rectifier module. The<br />SCR conducts only when gate gets positive pulse. This pulse is given by pulse<br />transformer, which gets pulses from firing card. Firing card gives pulse to the<br />respective RYB SCR only when their phases are positive maximum. The freewheel<br />diode is incorporated to protect the SCR and diodes from back emf when supply to<br />coils is cut off due to collapsing magnetic field.<br />The filter is provided to smoothen the ripple output and the bleeder is used for<br />voltage regulation. It gives improved voltage regulation and acts as a minimum load.<br />Also it keeps the SCR in conducting state by drawing the minimum current which is<br />higher than the SCR holding current. Thus there is always output voltage irrespective<br />of load.<br />DC CT is used for limiting output current. It works on principle of magnetic<br />amplifier. There is also provision for smooth rising of output voltage.<br />14. What is purpose of freewheeling diode and DC filter circuit in the charger?<br />Freewheeling diode is used to protect the semiconductor components used in the<br />charger from the back emf, which is induced in the inductive coils of relays when the<br />supply to the relays is cut off. The magnetic field in the relays collapses and induces<br />high voltage in reverse direction. This emf is shunted by the freewheeling diode,<br />which is connected in reverse bios with the output.<br />DC filter is used to smoothen the output, which has ripple. Ripple frequency is same<br />as system frequency for half wave rectifier and 2 times of system frequency for full<br />wave rectifier. The filter, which is a capacitor, will oppose any change in voltage.<br />Thus the ripple will not be allowed to come to zero.<br />Question and answers Electrical Maintenance Unit<br />- 125 -<br />Electrical equipment fundamental<br />1. Why indoor switchyard is provided in MAPS?<br />The salt contamination in the switchyard is avoided by providing indoor switchyard<br />in MAPS. Because the plant is just 300 metres away from the seashore and the<br />atmosphere is saline. This salt will get deposited on the insulators and on the<br />conductors used in the switchyard. Due to this deposition insulators may fail to<br />unnecessary trip the system and conductor used must be copper for better<br />performance instead of low cost aluminum. So to avoid unnecessary trip and to have<br />low cost of installation and spare parts of aluminium indoor switchyard is used.<br />2. What do ABCB and ACB mean?<br />ABCB – Air blast circuit breaker.<br />ACB – Air circuit breaker.<br />3. What do you mean by frequency?<br />The number of cycles per second is called the frequency.<br />4. State the voltage and current relation in star and delta connection.<br />In star connection line current is equal to the phase current and line voltage is √3<br />times that of phase voltage.<br />In delta connection line voltage is equal to the phase voltage and line current is √3<br />times that of phase current.<br />5. In DC motor what is the relation between speed and field flux?<br />Speed of a DC motor is inversely proportional to the field flux.<br />6. What is the difference between self-excited and separately excited DC generator?<br />Self-excited generator: In a self-excited generator the field winding is excited by an<br />external DC source like a battery etc.<br />Separately excited generator: In a separately excited generator the field poles have<br />some residual magnetism. When the armature is rotated a small emf is induced in it.<br />This is fed to the field winding and if the current direction is such that it adds the<br />residual magnetic flux to the field winding and the field strength is increased. The<br />more emf in the armature, which is again fed to the field winding and goes on till the<br />generator builds up voltage.<br />Question and answers Electrical Maintenance Unit<br />- 126 -<br />Power and control cables.<br />1. Define conductor.<br />Conductor: A material of low resistance used to transmit electrical energy. Examle<br />wires, cables, bus bars etc.<br />2. Define unprotected insulated wire.<br />Unprotected insulated wire: Unprotected insulated wire is which the insulation of the<br />wire is not covered by a protective sheathing to prevent it from mechanical damage.<br />3. Define cable.<br />Cable: One or more insulated conductor enclosed in a protective mechanical<br />sheathing of either GI wire or GI strip or aluminium to protect the insulation from<br />mechanical damage.<br />4. Define insulated wire.<br />Insulated wire: A conductor or multi-stranded conductor which has a insulating<br />material on it is called a insulated wire.<br />5. Explain briefly about armouring for an under grounded cable.<br />Armouring is required to protect the cores from mechanical damage.<br />6. Explain briefly about grounding of cable trays.<br />Cable trays are grounded because to avoid any shocks to personnel incase of leakage.<br />A grounding wire runs at the side of tray through a parallel groove clamp through out<br />the length of the cable tray. If trays are one above another we can loop up the<br />grounding wire to the tray below. This saves extra ground wire.<br />7. What are the differences between the power and control cable?<br />Power cable: It is used for supplying current to load. It is of larger current carrying<br />capacity available in single core, 2 cores, 3 cores, 3 cores, and 4 cores. Single core<br />is available upto 1000 mm2. Usually power cables are of aluminium. These cables<br />are graded for higher voltages and possess more cross section area.<br />Control cable: Control cables are used for control purposes for logics, indication or<br />annunciation etc. These are of lower current carrying capacity and voltage grading is<br />also less. These are of less cross sectional area are available in pairs of 2,5,10,25,50<br />etc.<br />8. What is the purpose of using corrosion inhibiting compound?<br />It is used for aluminium conductors while crimping to a lug or ferrule. It prevents<br />corrosion of aluminium conductor due to oxidation and due to saline atmosphere.<br />9. Why aluminium armouring for single core 1000 mm2 is used?<br />Question and answers Electrical Maintenance Unit<br />- 127 -<br />Aluminium armouring for single core 1000 mm2 is used so that heating will not take<br />place due to the flux around the conductor, as the aluminium is a non-magnetic<br />material.<br />Question and answers Electrical Maintenance Unit<br />- 128 -<br />10. What is resistance of copper compare to the aluminium?<br />Copper is less resistive than aluminium.<br />11. What is applied over the steel tape armour of PILC cable as serving?<br />Bituminous covered jute.<br />12. What is used as insulation for PILC cable?<br />Impregnated paper.<br />13. A small quantity of impurity reduces how much of conductivity of copper?<br />35% of conductivity will be reduced due a small impurity in the conductor.<br />14. Why cast aluminium tri-foil clamp is used in single core cables laying?<br />When three conductors are clamped together the fluxes around the conductors are get<br />cancelled<br />15. What are the parts of a cable gland?<br />a. Check nut.<br />b. Nipple.<br />c. Metal washer.<br />d. Neoprene rubber.<br />e. Metal washer.<br />f. Compression nut.<br />16. What are the advantages of PVC insulated cable?<br />a. Plumbing is not required. Joints can be made easily.<br />b. As PVC is light the injury caused to it while laying is less.<br />c. It is corrosion resistant.<br />d. It has high fire retarding property.<br />e. It does not break down even if moisture enters.<br />Question and answers Electrical Maintenance Unit<br />- 129 -<br />Basic electronics<br />1. How many types of logic gates are there?<br />a. OR gate.<br />b. AND gate.<br />c. NAND gate.<br />d. NOR gate.<br />e. Inverter gate.<br />f. Exclusive OR gate.<br />g. Equivalent gate.<br />Question and answers Electrical Maintenance Unit<br />- 130 -<br />Mechanical equipment fundamentals<br />1. Why feed valves are used?<br />Feed valves are used to prevent back flow.<br />2. What are the functions of a heat exchanger?<br />The function of heat exchanger is to transfer heat efficiently and economically<br />between two fluids. Heat is transferred from the fluid, which has higher temperature<br />to the fluid, which has lower temperature. The modes of heat transfer are,<br />a. Conduction – Heat transfer in solids by momentum of molecules.<br />b. Convection – Heat transfer in liquids by movement of molecules.<br />c. Radiation – Heat transfer by energy waves.<br />There are three types of flow in heat exchangers<br />a. Parallel flow.<br />b. Counter flow.<br />c. Transverse flow.<br />3. Why baffle plates are used in heat exchanger?<br />Baffle plates are used in heat exchanger so that the maximum heat can be transferred<br />and to avoid tube sagging.<br />4. How pumps are classified?<br />A. Centrifugal pumps – a. Single volute<br />b. Double volute<br />c. Diffuser type<br />d. Mixed flow<br />e. Axial flow<br />f. Turbine or regenerative type<br />B. Rotating pumps a. Screw type<br />b. Gear type<br />c. Vane type<br />C. Reciprocating pumps a. Piston<br />b. Plunger<br />c. Bucket<br />5. What you mean by cavitation and NPSH?<br />Cavitation: Bubbles form in the liquids whenever there is pressure reduction inside<br />the pump. These bubbles collapse when they approach high-pressure areas damaging<br />pump internals. This is called the cavitation.<br />NPSH: Net Positive Suction Heat. It is the head available at the eye of the impeller<br />corrected to vapour pressure.<br />Question and answers Electrical Maintenance Unit<br />- 131 -<br />6. Name the functions of the valve?<br />a. ON and OFF service.<br />b. Throttling or regulating liquid flow.<br />c. Avoid back flow.<br />d. Regulating pressure.<br />e. Relieving pressure.<br />7. What are the advantages of butterfly valve?<br />a. Less holdup.<br />b. No support needed.<br />c. Any actuator can be used.<br />d. Quick opening and closing.<br />e. Less space required.<br />f. Used for low-pressure low temperature and large pipelines.<br />Question and answers Electrical Maintenance Unit<br />- 132 -<br />Instrumentation fundamentals<br />1. What are the classifications of industrial instrumentation?<br />a. Information gathering instrumentation.<br />b. Regulating instrumentation.<br />c. Protective instrumentation.<br />2. What are the units of pressure?<br />Pounds / inch2 and kg / cm2.<br />3. What is the use of BAROMETER?<br />Barometer is used to measure atmospheric pressure.<br />4. What are the methods used to measure the flow?<br />Mechanical (float) type and ultrasonic type methods are used to measure the flow.<br />5. What is the equivalent of atmospheric pressure?<br />One atmospheric pressure is equal to 10 meters of water column or 760 mm of<br />mercury.<br />6. What method is used to measure the level?<br />Bubbler method is used to measure the level.<br />7. State some elements of pressure measurement.<br />Manometer, Diaphragm gauges, Bellows, Strain gauges etc.<br />8. State some elements of flow measurement.<br />Orifice, Venturi tubes, flowrator (rotameter) etc.<br />9. State some thermocouple.<br />Copper – constantan, Iron – constantan.<br />10. What is the purpose of instruments?<br />The purpose of instruments is to measure, safeguard the process for efficient plant<br />operation.<br />Instruments are very accurate and fast acting. This accuracy and speed is not possible<br />by human. Also in some places there may be too much heat for man to work or some<br />where there may high radiation field. In such cases instruments provide remote<br />operation.<br />11. What is primary element and what should be its response?<br />Primary element is one, which senses the condition of process, and converts it to<br />some other form, which can be measured accurately. Example in a bourden gauge<br />the pressure if changed to the uncoiling (displacement), which can be measured.<br />Question and answers Electrical Maintenance Unit<br />- 133 -<br />The response of primary element is that it should convert the condition in to some<br />other form, which can be interpreted and measured easily.<br />Question and answers Electrical Maintenance Unit<br />- 134 -<br />12. Name some process variables, which are appropriate for our plant with examples.<br />Process variables with examples, which are appropriate to our plant, are<br />a. Flow – flow of D2O in PHT system.<br />b. Temperature – temperature of coolant in PHT system.<br />c. Level – moderator level.<br />d. Speed – speed of turbine.<br />e. Voltage – voltage generated by main generator.<br />f. Neutron flux – number of neutrons produced in reactor during operation.<br />g. pH – pH of moderator.<br />13. What is use of 2/3 logic in our plant?<br />All our protection instruments (system) are triplicated to have following uses.<br />a. To increase system integrity.<br />b. To decrease faulty trips.<br />c. Maintenance can be done on one protective instrument without shutting down the<br />whole system.<br />We don’t want our plant (reactor) to trip just because one instrument failed. So we<br />have triplication (2/3 logics) in protection instruments. The trip signal will pass if<br />only two out of three switches operate. Of only one operates there will be no trip.<br />This logic is used to trip the reactor in our plant.<br />14. What is resistance temperature detector (RTD) and mention some examples?<br />Resistance temperature detector is an instrument, which is used to measure<br />temperature. This uses the property that the resistance of a metal changes (increases<br />of decreases) with temperature. This is very accurate. These will be a wire, which<br />will senses the temperature and changes its resistance as the temperature changes.<br />This varying in resistances if measures by an external electronic or electrical circuit<br />calibrated to measure temperature.<br />Different types of RTD’s are Platinum, copper, nickel.<br />15. What is recorder and how it is useful to our plant?<br />Recorder is an instrument, which gives instantaneous values as well as records the<br />values.<br />Recorder can show us where a fault has occurred if reactor trips. It also gives us past<br />information recorded in it. It saves human effort because an operator cannot sit and<br />record the information required and it is very difficult task to an operator.<br />Question and answers Electrical Maintenance Unit<br />- 135 -<br />Fire fighting<br />1. How combustion takes place?<br />For combustion to take place three elements are needed. They are fuel, heat and<br />oxygen. This is called the triangle fire. Combustion can not survive without these<br />three. Remove any one of them, combustion ceases to take place. So wherever fuel,<br />oxygen and heat is there together combustion takes place.<br />2. How many types of extinction media’s are used in fire fighting?<br />a. Sand.<br />b. Water.<br />c. Foam.<br />d. Carbon – di – oxide.<br />e. Dry chemical powder.<br />f. Halons.<br />3. What are the classifications in fire?<br />a. Class A – Ordinary fire like burning of paper, wood etc.<br />b. Class B – Oil fire like burning of petrol, diesel, LPG etc.<br />c. Class C – Gas and dust fire like burning of butane, acetone, natural gas etc. and<br />burning of dust like uranium dust, sodium dust etc.<br />d. Class D – Metal fire like burning of uranium, thorium, sodium etc.<br />e. Class E – electric fire example transformer or switchgear fire etc.<br />4. How many types of fire extinguishers are there and state their suitability?<br />a. Soda acid type – suitable for Class A type of fires.<br />b. Foam type – suitable for Class A and Class B type of fires.<br />c. Carbon-di-oxide type – suitable for Class B, Class C and for Class E type of fires.<br />d. Dry chemical powder – suitable for Class B, Class C, Class D and Class E fires.<br />e. Halons BCF (bromo chloro difluoro methane) – suitable for Class A, Class B,<br />Class C and Class E types of fires.<br />5. At what areas of risk the Co2 flooding system, mulsifyre systems are provided?<br />Co2 flooding system is provided in diesel generator and turbine oil tank area.<br />Mulsifyre system is provided in generator transformer, start up transformer and unit<br />transformer areas.<br />6. What are the equipments kept inside the hose boxes?<br />a. Double female adapter (1 No).<br />b. Delivery hose pipe (50 feet – 2 Nos).<br />c. Branch pipe (1 No).<br />d. Valve wheel (1 No).<br />e. A hose box key (situated in a cabinet at side of hose box).<br />Question and answers Electrical Maintenance Unit<br />- 136 -<br />7. How water is used in a fire?<br />Water is used as a cooling effect in a fire.<br />Question and answers Electrical Maintenance Unit<br />- 137 -<br />8. How foam is used in a fire?<br />Foam is used as a blanketing effect in a fire.<br />9. Which extinguisher you use for electronic equipment fire?<br />Co2 or DCP type fire extinguisher can be used on fire involving electronic<br />equipments.<br />10. What you mean by starvation method?<br />Starvation method means elimination of fuel from the fire.<br />11. What is the name of powder used in Dry Chemical Powder extinguisher?<br />Sodium-bi-carbonate.<br />12. What you mean by cooling method?<br />Cooling method means elimination of heat from the fire.<br />13. What you mean by blanketing method?<br />Blanketing method means elimination of oxygen from the fire.<br />14. Why Co2 is used on Class E fire?<br />Co2 is a non-conductor of electricity.<br />Question and answers Electrical Maintenance Unit<br />- 138 -<br />First aid<br />1. What is the golden rule of first aid?<br />Do first thing first, artificial respiration, stop bleeding and treat shock. Do not<br />attempt too much, reassurance, avoid crowing and transfer.<br />2. What do you mean by diagnosis?<br />Determining the nature and courage of a disease.<br />3. For a bleeding what is the first aid?<br />Take care to stop the bleeding by giving pressure.<br />4. What is the first aid for bone injury?<br />Support the injured part and painkillers.<br />5. What is the first aid for burn cases?<br />Water, warm fluids should be given when the victim is conscious.<br />6. How we can differentiate the bleeding from artery and vein?<br />By the colour of the blood which is bleeding.<br />7. What is the first aid for chlorine inhaled victims?<br />Remove the victim from the source, fresh air and artificial respiration if necessary.<br />8. What is the first aid for dog bite?<br />Suck the wound and spite out.<br />9. What is the first aid for snakebite?<br />Bath the wound and constrictive bandage. Give warm drinks and rest to the patient<br />and artificial breathing if necessary.<br />Question and answers Electrical Maintenance Unit<br />- 139 -<br />D20 handling<br />1. What is the instrument name used for accurate measurement of IP?<br />Infra red spectro photometer.<br />2. How D20 vapour is recovered?<br />Dryer recovers D20 vapour.<br />3. What is the amount of D20 used in moderator?<br />140 tonnes.<br />4. Name the heavy water plants in India.<br />a. Nangal.<br />b. Kota.<br />c. Baroda.<br />d. Tuticorn.<br />e. Talcher.<br />f. Thal (under construction).<br />g. Hazira (under construction).<br />h. Malugum (under construction).<br />5. Define reactor grade and down graded D20.<br />Reactor grade D20: If the isotopic purity of a given D20 is more than or equal to<br />99.7% then the D20 is reactor grade D20.<br />Down grade D20: If the isotopic purity of a given D20 is less than 99.7% then the<br />D20 is downgraded.<br />6. What precautions should be taken while working in high tritium areas?<br />Use respirators, plastic suits, VP suits if concentration of tritium is very high. Avoid<br />getting hurt while working because tritium may go through the skin by sweat to the<br />blood. If by chance there is tritium intake in the body drink lots of fluids.<br />7. Why spillage of D20 is to be avoided?<br />Cost consideration: D20 very costly and very valuable. Cleaning of spillage also cost<br />and extra manpower to be deployed.<br />Tritium hazard: D20 contains tritium, which when spilled becomes tritiated vapour<br />and finds access through human body. Tritium is a radioactive material. It is a beta<br />emitter.<br />8. What is ice plugging?<br />If there is a need to repair a valve of D20 PHT system, there are no other valves to<br />shut of D20. So we use plastic bags on pipes and it has dry ice. Then liquid nitrogen<br />is poured inside the bag. Due to the low temperature the D20 inside pipeline<br />solidifies preventing any flow of D20 when valve is removed. This is called the ice<br />plugging.<br />Question and answers Electrical Maintenance Unit<br />- 140 -<br />Question and answers Electrical Maintenance Unit<br />- 141 -<br />9. Name the methods by which D20 leak can be detected.<br />a. By beetles.<br />b. D20 losses through stack monitoring.<br />c. Tritium monitoring.<br />d. In heat exchangers the leakage can be found by taking samples of process water<br />from all heat exchanges and counting the tritium activity.<br />10. Name the D20 recovery methods.<br />a. Manual mopping and vacuum cleaning.<br />b. Active drainage recovery.<br />c. Vacuum mopping recovery.<br />d. Dryers recovery.<br />e. Vapour recovery.<br />11. Name the features for reducing D20 leaks.<br />a. Reduce valves and fittings in the pipelines.<br />b. Use welded joints instead of flanged joint.<br />Question and answers Electrical Maintenance Unit<br />- 142 -<br />Nuclear reactor theory<br />1. State the law of conservation of mass and energy.<br />Mass and energy are interchangeable. When mass is lost there is a substantial gain in<br />energy and when energy is lost there is a increase in mass.<br />Energy = mass*C2<br />E = MC2<br />Where C is the conversion factor = 3*1010 (velocity of light)<br />C2 = 9*1020<br />2. Which of the following material is good moderator? Why?<br />a. H2 b. D2 c. H2O d. D2O e. Beryllium f. Graphite.<br />The functions of a good moderator are<br />a. It has to thermalise neutrons effectively.<br />b. There should be only a minimum absorption of neutrons.<br />c. It should not be toxic.<br />d. It should not be inflammable.<br />In case of Hydrogen (H2) though it is a effective sheatherer it cannot be used as a<br />moderator because it is a gas and there is a lot of distance between atoms. A neutron<br />cannot meet the nucleus in a definite manner.<br />For D2 also the same problem as this is also gas and it cannot be used as a moderator.<br />In case of H2O it is a good scatterer but is absorbs neutrons. Its moderation ratio is<br />72. So it cannot be used as a moderator.<br />In case of D2O though it is not as effective scatterer as that of H2O it has minimum<br />absorption of neutrons. It has a moderating ratio of 21000. This is an ideal<br />moderator.<br />In case of Beryllium it is a toxic material. Therefore cannot be used.<br />Graphite absorbs neutrons and is inflammable and therefore cannot be used.<br />So from the above statements the D2O is the good moderator material.<br />3. The activity of an Iodine – 131 is 10 curies. After how many half-lives will it come<br />down to 625 millicuries?<br />Activity of an Iodine – 131 = 10 curies<br />To find number of half-lives for coming it to 625 millicuries,<br />10 * 1st  = 5 (1st half-life)<br />5*  = 2.5 (2nd half-life)<br />2.5 *  = 1.25 (3rd half-life)<br />1.25 *  = 0.625 (4th half-life)<br />So during 4th half-life the Iodine – 131 reduces to 625 millicuries.<br />Question and answers Electrical Maintenance Unit<br />- 143 -<br />Question and answers Electrical Maintenance Unit<br />- 144 -<br />4. What do Atomic number and mixture mean?<br />Atomic number: The atomic number of an atom is the number of protons in that<br />atom.<br />Example – Hydrogen has one proton and its atomic number is one (1).<br />– Uranium has 92 protons and its atomic number is 92.<br />Mixture: It is a substance formed by different elements and these elements can be<br />separated by physical methods.<br />Example – Air is a mixture of oxygen and nitrogen and these can be separated by<br />physical methods.<br />5. What is the weight of a proton?<br />1.00759 amu (atomic mass unit)<br />6. What is the energy of a thermal neutron?<br />0.025 eV (energy volt)<br />7. What is the charge of an alpha particle?<br />2+.<br />Question and answers Electrical Maintenance Unit<br />- 145 -<br />Nuclear system<br />1. What is the purpose of moderator D2O?<br />The purposes of moderator D2O are,<br />a. To thermalise neutrons to maintain criticality.<br />b. Emergency core cooling when PHT fails.<br />c. Structural cooling.<br />2. What is the cooling water used in moderator heat exchanger during normal<br />operation? And during class IV failure?<br />During normal operation process LP water is used in moderator heat exchangers,<br />which transfers heat to seawater. During class IV failure firewater is used for the<br />moderator heat exchanger.<br />3. During loca how the cooling of fuel bundle is achieved?<br />During loca there is provision for taking D2O from the moderator system, which is<br />connected to PHT system for fuel cooling and there is one more option for the<br />cooling of the fuel from the fire water system.<br />4. How coolant flow is maintained in the event of loss of power to the PHT system?<br />When the PHT power fails, it takes two minutes for DG to come to full power. The<br />circulation is maintained by flywheel, which increases the de-acceleration and<br />maintains the flow for two minutes.<br />If there is a station blackout the flow is maintained by thermo-symphoning by which<br />more denser cold water comes down and less denser hot water goes up by convection<br />method.<br />5. What are the materials in contact with PHT system?<br />Piping – carbon steel.<br />Pressure tubes – zircalloy-2<br />Fuel cladding – zircalloy-2<br />End shield – stainless steel. Etc.<br />6. What are the main futures of PHT system?<br />a. Impeller – To reduce the water flow.<br />b. Self-injection Hx – Cools the water in case of leakage.<br />c. Gland supply – Cools the seals.<br />d. Primary and secondary seals – sealing the pump.<br />e. Thrust bearing – To take the axial thrust.<br />7. What are the purposes of bleed condenser?<br />The purposes of bleed condenser are,<br />a. To cool the bleed water, which goes to bleed cooler.<br />Question and answers Electrical Maintenance Unit<br />- 146 -<br />b. To provide cool hot water to the PHT purification system.<br />Question and answers Electrical Maintenance Unit<br />- 147 -<br />8. What is header level control?<br />For the purpose of maintenance of PHT pipe valves, boiler inlet valve the level of the<br />coolant should be below the valve to prevent the coolant coming out of the system.<br />This is called the header level control and achieved by means of manual operation of<br />valves and shutdown cooling pump.<br />9. In MAPS Unit # 1 end-shield cooling system has got heaters. Why?<br />Unit # 1 end-shield is made of nickel steel, which had a nil ductility temperature of<br />-100°C while commissioning. After 30 years of operation this will rise to +32°C,<br />because of radiation exposure. To avoid the failure of end shield of MAPS Unit # 1<br />the temperature of the end-shield should be maintained at 68°C always. MAPS 1<br />end-shield cooling system is operating at elevated temperature. But in MAPS Unit #2<br />this problem is not existed because in this unit end-shields are made up of stainless<br />steel.<br />Question and answers Electrical Maintenance Unit<br />- 148 -<br />Reactor general<br />1. What are V1 and V2? How they are connected? What is the harm in opening F/M<br />vault door during reactor operation?<br />The reactor building is divided into two areas. They are V1 and V2.<br />V1 – Dry volume area.<br />V2 – Wet volume area.<br />V1 area includes F/M vault, boiler room, and entire dome area. Rest of the areas in<br />the reactor building is V2 area. V1 and V2 are connected by vent shaft through<br />suppression pool.<br />F/M vault is a V1 area and F/M maintenance bay is a V2 area. Usually when there is<br />an accident in F/M vault the pressure is relieved through boiler room through<br />suppression pool and the uncondensed gases to V2 area. When the reactor is in<br />operation and if we open the F/M vault door, suppose of there is an accident in F/M<br />vault the pressure released directly goes to F/M maintenance area, which is a V2<br />area. Thus pressurizing the entire building.<br />2. What is the purpose of inlet manifold inside calandria? What is the material used for<br />that?<br />The purpose of inlet manifold is to introduce heavy water to the calandria with low<br />velocity to avoid mixing. That is stratified flow of D2O is obtained b inlet manifold.<br />Thus the temperature is kept minimum. It is made of zircalloy.<br />3. Why bi-directional flow is chosen for PHT system?<br />Bi-directional flow is chosen for PHT system because,<br />a. Uniform temperature gradient is facilitated so there will be no differential thermal<br />expansion.<br />b. It facilitates fuelling even when the reactor is working, which facilitates uniform<br />neutron flux and this intern gives rise to maximum fuel burns up.<br />4. What is the purpose of end-shield?<br />The purposes of end-shield are,<br />a. To permit access to F/M vault during shut down.<br />b. To provide tight clamping for fuelling machines.<br />c. To support the calandria tubes and also system.<br />Question and answers Electrical Maintenance Unit<br />- 149 -<br />Station grounding<br />1. What is grounding?<br />It is an electrical connection with the general mass of earth through an earth<br />electrode.<br />2. What is difference between earthing and grounding?<br />Both have same meaning. The term earthing is used in U.K. and grounding in U.S.A.<br />ground means earth.<br />3. What are types of grounding?<br />There are two types<br />a. System grounding.<br />b. Equipment grounding.<br />4. What does mean by system?<br />Grounding of neutral point of equipment is called system grounding. For instance<br />grounding of generator neutral, transformer neutral etc.<br />5. What does mean by equipment grounding?<br />Grounding of non-current carrying metallic parts is called equipment grounding. For<br />instance no-current carrying parts include the following:<br />a. Motor body, switchgear metal enclosure, transformer tank, conduits of wiring etc.<br />b. Support structures, tower, poles etc. in the neighborhood of electrical circuits.<br />c. Sheath of cables.<br />d. Body of portable equipment such as iron, oven, etc.<br />6. What is the important of system grounding?<br />It is important because:<br />a. Earth fault protection is based on the method of neutral earthing.<br />b. System voltage during earth fault depends on neutral earthing.<br />c. It is a protection against arcing grounds, unbalanced voltages with respect to earth<br />and lighting.<br />7. What is the important of equipment grounding?<br />Equipment earthing ensures safety.<br />8. How safety could be ensured by equipment grounding?<br />In order to enumerate this, let us first find out the effects of current and voltage<br />developed during fault condition.<br />Question and answers Electrical Maintenance Unit<br />- 150 -<br />9. What is the permissible body current limit?<br />The magnitude and duration of current conducted through a human body at 50 Hz<br />should be less than those did that cause ventricular fibrillation.<br />(Ventricular fibrillation is considered to be the main cause of death due to electrical<br />shock). These below given data are also applicable for current limits to human body.<br />Current magnitude Physiological effect Description<br />1 mA Threshold of<br />perception<br />A current at which a person is just able to<br />detect a slight tingling in his hand or finger<br />1 – 6 mA Unpleasant to sustain This is often termed as let go currents. Do not<br />impair the ability of a person holding an<br />energised object to control his muscles and<br />release it.<br />6 – 9 mA Threshold of muscular<br />contraction.<br />These are threshold values, since 10.5 mA<br />current and 16 mA current are the let go values<br />for women and man respectively.<br />9 – 25 mA Muscular contraction May be painful and can make it hard or<br />impossible to release energised objects grasped<br />by the hand.<br />25 – 60 mA Muscular contraction Make breathing difficult.<br />60 – 100 mA Ventricular fibrillation Ventricular fibrillation, stoppage of heart or<br />inhibition of respiration might occur and cause<br />injury or death if time is more than 1 sec.<br />Hence the grounding equipment shock current can be kept below the value sufficient<br />to cause injury or death by lowering the step and touch potential.<br />Question and answers Electrical Maintenance Unit<br />- 151 -<br />10. How fibrillation current functions?<br />Fibrillation current is actually function of individual body weight.<br />For 50 kgs body weight: fibrillation current (IB) = 0.116/ª ts (Limited to 0.03 – 3<br />sec. Range)<br />Where ts = duration of current exposure in sec.<br />Note = Above equation results = 116 mA for 1 sec. and 367 mA for 100 sec.<br />For 70 kgs body weight: fibrillation current (IB) = 0.157/ª ts<br />Note = Above equation results = 157 mA for 1 sec. and 496 mA for 100 sec.<br />Above times are very - very important from the point view of clearing the fault.<br />Above limit dictates that grounding should e such that current magnitude through<br />human body should not increase the specified values.<br />In order to ensure above following have been done.<br />1. Current conductor have been burried in ground<br />a. At the depth of 600 mm in switchyard. Depth 600 mm is normally selected<br />because of freezing or drying out, the Resistivity of upper layers could vary<br />with seasons, while the Resistivity of lower soil layers remains nearly<br />constant.<br />b. Horizontal grid conductors are more effective in reducing the danger of high<br />step and touch voltages on the earth surface by creating equipotential surface<br />during fault conditions.<br />c. At the depth of 800 mm else where. Here depth is kept more because to care<br />for under grounding services. Example laying of power cables, drainage etc.<br />2. 25-mm dia copper rod electrodes have been driven in soil.<br />a. Upto 5 meters depth in 220 kV switchyard.<br />b. Upto 3 meters elsewhere.<br />Why only 5 meters and 3 meters depths have been selected is that the<br />resistance is diminishes rapidly with the first few feet of driving, but less so at<br />depths greater than 2 to 3 meters in soil of uniform resistivity.<br />These lengths are adopted in selecting the ground electrodes.<br />3. 4-inch layer of gravel in 220 kV switchyard has been used. Purpose of using<br />gravel is by doing steps 1,2 above tough and step potential are computed and<br />compared with tolerable potential and found as given below.<br />Potential Computed value Tolerable value<br />Tough 550 V 665V<br />Step 2.a switchyard with crushed rock surface 230V 2165 V<br />Step 2.b elsewhere with natural soil 166V 168.5 V<br />Question and answers Electrical Maintenance Unit<br />- 152 -<br />11. Why grounding is necessary?<br />The purpose of grounding is to maintain the surface under and around a station ate as<br />nearly zero potential as possible with reference to absolute earth so that operating<br />staff who walk in the station yard and tough equipments are ate earth potential and<br />when faults occur there is safety to staff and equipments.<br />12. What are the harms of under grounded system?<br />a. Step and tough potential will increase more than maximum tolerable value.<br />b. Under single line to ground fault the voltage to earth of the two healthy phases<br />rises from their normal phase to neutral voltage to full line voltage, which may<br />result in insulation break down.<br />c. The capacitive current in two healthy phases increases ª3 times the normal value.<br />d. The capacitive current in the faulty phase is 3 times its normal value.<br />e. Experience shows that capacitive current in excess of 4 amps may be sufficient to<br />maintain an arc in the ionized path of the fault and this persistent arc phenomenon<br />is called ARCING GROUND, which ultimately cause high voltage build up.<br />Some time these voltage builds up to 5 to 6 times its normal value, which results<br />in break down of insulation.<br />f. Being fault current low, it is difficult to isolate fault.<br />13. How system grounding and equipment grounding achieved?<br />System grounding is obtained by grounding the neutral through resistance, through<br />transformer and through effective or solidly grounding.<br />Equipment grounding is obtained by Grounding of non-current carrying metallic<br />parts equipment. For instance no-current carrying parts include the following:<br />a. Motor body, switchgear metal enclosure, transformer tank, conduits of wiring etc.<br />b. Support structures, tower, poles etc. in the neighborhood of electrical circuits.<br />c. Sheath of cables.<br />d. Body of portable equipment such as iron, oven, etc.<br />14. What does mean by grounding electrode, grounding system, and grounding<br />resistance?<br />Grounding electrode: A conductor driven in the earth and used for collecting ground<br />current from or dissipating ground current into the earth.<br />Grounding system: Comprises all interconnected grounding facilities in a specific<br />area.<br />Grounding resistance: The resistance offered by the ground when power frequency<br />current is discharged to the ground through a particular grounding electrode or<br />grounding system.<br />Question and answers Electrical Maintenance Unit<br />- 153 -<br />15. How grounding resistance could be measured?<br />There are few methods, which can give approximately true value. These are<br />described below.<br />a. Fall of potential method: This method is applicable for small grid or sub station<br />where induction effect of voltage is less.<br />b. Measurement of earth resistance by 61.8% distance rule:<br />c. Alternate – 1 of fall of potential method: This method is influenced by induction<br />effect.<br />d. Alternate – 2 of fall of potential method:<br />16. How value of grounding resistance could be kept constant?<br />While measuring of grounding resistance is more than computed design value 0.11Ω,<br />then following are recommended to reduce it. Add in water the following highly<br />conductive substances and pour into treated pit.<br />a. Sodium chloride (Nacl), known as common salt.<br />b. Calcium chloride (Ca CL2)<br />c. Sodium carbonate (Na2 CO3)<br />d. Copper sulphate (Cu SO4)<br />e. Soft choke and<br />f. Salt and charcoal in suitable proportions.<br />17. What is the effect of moisture content on earth resistivity?<br />The moisture content is expressed in percent by weight of dry soil. Dry earth weights<br />about 1440 kg per cubic meter and thus 10% moisture content is equivalent to 144 kg<br />of water per cubic meter of dry soil. So about 20% moisture, the resistivity is very<br />little affected. Below 20%, the resistivity increases very abruptly with the decrease in<br />moisture.<br />18. What is the effect of salt content in moisture on resistivity?<br />The resistivity decreases and the salt content is expressed in percent by weight of the<br />contained moisture. It will be noted that the curve flattens off at about 5% salt<br />content and a further increase in salt content gives little decrease in the soil<br />resistivity.<br />19. What is the effect of temperature on earth resistivity?<br />The temperature co-efficient of resistivity for soil is negative, but it is negligible for<br />temperature above freezing point. Below 0°C the water in the soil begins to freeze<br />and introduces a tremendous increase in the temperature co-efficient, so that as the<br />temperature becomes lower the resistivity rises enormously.<br />20. What does mean by neutral floating or neutral displacement?<br />Question and answers Electrical Maintenance Unit<br />- 154 -<br />When a ground fault occurs, there is a tendency of neutral shift with consequent<br />change in voltage on the un-faulted phases. This phenomenon is called neutral<br />floating or neutral displacement.<br />Question and answers Electrical Maintenance Unit<br />- 155 -<br />21. Why grounding of power cable is needed? How it should be done?<br />a. The magnetic fluxes produced by the three phases in a multi core power cable<br />almost cancel put each other, since the vector sum of these currents at any instant<br />is zero and practically there is no residual magnetic flux around the cable.<br />In case of single core cable, the magnetic flux induces the voltage in the metallic<br />sheath.<br />b. When the cable conductor is carrying alternating current, for safe and reliable<br />operation, the metallic sheath must be grounded. If the metallic sheath is at one<br />end the potential of the unearthed end could be much above the earth potential. If<br />both ends are grounded, a circulating current is induced in the metallic sheath.<br />c. The maximum acceptable induced voltage under normal load current operation is<br />limited by corrosion and safety considerations.<br />d. Code of practice of earthing (IS 3043) as well as electricity council London<br />recommended permissible induced voltage level of 65 Volts.<br />Hence keeping above all points in mind metallic sheath and armour of all multi core<br />power cables shall be earthed at both end equipment and switchgear end. Sheath and<br />armour of single core power cable shall be earthed ate switchgear end only. The<br />sheaths of shielded control cables should be grounded at both ends to eliminate<br />induced potentials.<br />22. In 220 kV switchyard why lightning arrestor should be properly grounded?<br />a. During lightning, surges should be discharged to ground, otherwise it will<br />puncture the equipment insulation and it is possible only when lightning arrestor<br />is grounded properly.<br />b. In order to make it effective, the ground terminal of lighting arrestor should be<br />connected direct to the tank of transformer. This will eliminate voltage build up<br />due to earth resistance. For example for each ohm of earth resistance the voltage<br />build up for 5000 Amps discharge current is 5 kV. Soil resistivity a should be<br />minimum and may be it is 3.5 ohm per meter.<br />23. Why grounding mat is important near ground switch operating handle and<br />disconnecting switch operating handle?<br />Equipment operating handles deserve special attention because of the higher<br />probability for co-incidence of adverse factors. For example,<br />a. Hand operation equipment such as grounding switches and disconnecting<br />switches requires the presence of operator near a grounded structure at a point<br />where opening of an energised circuit can some times result in an arc to the<br />structure or perhaps mechanical failure and electrical break down of a switch<br />insulator. A large percentage of fatal accidents from voltage gradients are in fact<br />associated with operating handles. Hence in order to avoid above problems<br />following should be an additional safety factors:<br />Question and answers Electrical Maintenance Unit<br />- 156 -<br />1. Use closer mesh in the vicinity of operating handle area (150-mm approx.) and<br />operating handle shall be directly connected to the earthing mat.<br />2. Use higher resistance surfacing such as crushed rock or both in order to bring<br />down the values of touch potential and step potential.<br />Question and answers Electrical Maintenance Unit<br />- 157 -<br />24. Why fences grounding are important?<br />Because the most dangerous touch contacts involves and outside the fence are<br />usually accessible to the general public. In order to minimise the effect of step<br />potential and touch potential following two philosophies could be adopted.<br />a. Inclusion of the fence within the ground grid area and<br />b. Placement of fence outside the ground grid area – not safe to use.<br />With this effective area is increased and reduces ground grid resistance substantially<br />and maximum ground – grid voltage rise as well. In this case the perimeter conductor<br />of grid normally either follow the fence line, or parallel to it at a short distance about<br />0.5 m – 1.5 m outside. In either case, the perimeter ground conductor and fence are<br />bonded electrically at frequent intervals.<br />25. What are the specifications for procurement of grounding conductor and grounding<br />rods?<br />Grounding conductor, pad, rods etc. should have following specifications:<br />a. Copper : 91.8 to 94.9%<br />b. Zinc : 2.0 to 3.0%<br />c. Tin : 0.8 to 1.5%<br />d. Lead : 2.0 to 2.5%<br />e. Iron : 0.5 to 1.0%<br />Impurities must be limited to the percentage specified below:<br />a. Nickel : 0.3% maximum.<br />b. Antimony : 0.3% maximum.<br />c. Manganese : 0.04% maximum.<br />d. Phosphorous : 0.04% maximum.<br />26. Why copper is only preferred as material for grounding?<br />An advantage of use of copper is in addition to their high conductivity, has the<br />advantage of being resistant to underground corrosion. Copper is cathodic with<br />respect to other metals that are likely to be burried in the vicinity.<br />Disadvantages of use of copper are,<br />a. Grid of copper forms a galvanic cell with burried steel structures, pipes and any<br />of the lead based alloys that might be present in cable sheaths, it is likely be<br />hasten the corrosion of the latter.<br />b. Use of tinned copper conductor accelerates and concentrates the natural corrosion<br />of metal in small area however cell potential with respect to steel and zinc<br />reduces by about 50% and practically eliminates this potential with respect to<br />lead.<br />27. What should be the frequency of measurement of earth resistivity?<br />As per IS: 3043, 1987, measurement of earth resistivity should be carried out<br />annually or biannually and value should be recorded.<br />Question and answers Electrical Maintenance Unit<br />- 158 -<br />28. What should the statutory provision of earthing?<br />a. Earthing shall generally be carried out in accordance with the requirement of<br />India electricity rule 1956, as amended from time to time and the relevant<br />regulations of the electricity supply authority concerned.<br />b. All medium voltage equipment shall be earthed by two separate and distinct<br />connections with earth. In the case of high and extra high voltages, the neutral<br />points shall be earthed by not less than two separate and distinct connections with<br />earth, each having its own electrodes at the generating station or substation and<br />may be earthed at any other point provided no interference is caused by such<br />earthing. If necessary, the neutral may be earthed through suitable impedance.<br />c. As for as possible all earth connections shall be visible for inspection.<br />d. All connections shall be carefully made. If they are poorly made or inadequate for<br />the purpose for which they are intended, loss of life or serious personal injury<br />may result.<br />e. Each earth system shall be so devised that the testing of individual earth electrode<br />is possible. It is recommended that the value of any earth system resistance shall<br />be such as to confirm with the degree of shock protection desired.<br />f. It is recommended that a drawing showing the main earth connection and earth<br />electrodes be prepared for each installation.<br />g. No addition to the current carrying system, either temporary or permanent shall<br />be made which will increase the maximum available earth fault or its duration<br />until it has been ascertained that the existing arrangement of earth electrodes,<br />earth bus-bar etc. are capable of carrying the new value of earth fault current<br />which may be obtained by this addition.<br />h. No cut-out link or switch other than a linked switch arranged to operate<br />simultaneously on the earthed or earthed neutral conductor and the live<br />conductors, shall be inserted on any supply system. This however, does not<br />include the case of a switch for use in controlling a generator or a transformer or a<br />link for test purposes.<br />i. All materials fittings, etc. used in earthing shall conform to Indian standard<br />specifications, wherever these exist.<br />29. What maintenance of earth electrodes should be done?<br />The neighbouring soil to the earth electrode shall be kept moist where necessary, by<br />periodically pouring water through a pipe where fitted along with it or by pouring<br />water in the immediate vicinity of the earth electrode.<br />Periodical visual inspection of all earth electrodes connection wherever available,<br />shall be carried out to ensure their rigidity and other signs of deterioration.<br />Question and answers Electrical Maintenance Unit<br />- 159 -<br />30. In case new installation is to be done, what basic guidelines should be followed for<br />grounding?<br />a. Earthing conductors in outdoor areas shall be burried 500 mm below finished<br />grade level unless stated otherwise.<br />b. Minimum 6000 mm spacing between rod pipe electrode shall be provided unless<br />stipulated otherwise.<br />c. Earthing conductor around the building shall be burried in earth at a minimum<br />distance of 1500 mm from the outer boundary of building.<br />d. Earthing conductors embedded in the concrete floor of the building shall have<br />approximately 100-mm concrete cover.<br />e. Earthing conductors along their run on columns, beams, walls etc. shall be<br />supported by suitable cleats at intervals of 750 mm.<br />f. Earthing conductors crossing the road shall be either installed in hume pipes or<br />laid at greater depth to suit the site conditions.<br />g. Whenever earthing conductors cross underground service ducts, pipes, trenches,<br />under ground service ducts, pipes, trenches, tunnels, railway track etc. it shall be<br />laid 800 mm below them.<br />h. Earthing conductor shall be burried 1000 mm outside the switchyard fence. Every<br />alternate post of the fence and gates shall be connected to earthing loop by one<br />lead.<br />i. Each earthing lead from the neutral of the power transformer shall be directly<br />connected to a rod or pipe or plate electrode treated earth pit, which in turn shall<br />be connected to station earthing.<br />31. How much resistance human body has?<br />Resistance of internal body tissues (Not including skin) : 300 Ω.<br />Resistance of body including skin : 500 to 3000 Ω.<br />32. What is the effect of voltage frequency and current on resistance of the human body?<br />a. For touch voltages upto approximately 50V the value of impedance of the skin<br />varies widely with surface area of contact, temperature, respiration etc. even for<br />one person.<br />b. For higher touch voltages in order of approximately 50V to 100V the skin<br />impedance decreases considerably and becomes negligible when the skin breaks<br />down.<br />c. Wet hand contact resistance becomes very low at any voltage.<br />d. With increase in frequency, impedance of skin decreases.<br />Question and answers Electrical Maintenance Unit<br />- 160 -<br />33. What are the paths of current through the body?<br />A value of 1000 Ω is selected for the calculations that follows as representing the<br />resistance of a human body from hand to both feet and also from hand to hand or<br />from one foot to other foot.<br />Above paths includes vital organs such as heart.<br />a. Path from hand to foot is much more dangerous than foot to foot, since current<br />flow through heart during foot to foot current flow will be much less than the<br />current flow from hand to foot approximate ratio is 25:1<br />b. However deaths have occurred during foot to foot current flow. Hence can not be<br />ignored.<br />34. What are the effects of re-closure shock?<br />During re-closure, when fault is persisting a person might be subjected to the first<br />shock which would not permanently injure him, but would upset and disturb him<br />temporarily.<br />Next, a single fast automatic re-closure could in a second shock initiated within less<br />than 500 ms from the start of first. It is this second shock, occurring after a relatively<br />short interval of time before the person has recovered, that might cause a serious<br />accident. With manual re-closure the possibility of exposure to a second shock is<br />reduced since the time interval may be substantially greater.<br />35. State DC/AC equivalent factor (K).<br />Ratio of direct current (DC) to its equivalent rms value of alternating current (AC)<br />having the same probability of inducting ventricular fibrillation.<br />K = I DC fibrillation / I AC fibrillation (rms).<br />K = 3000 mA / 100 mA<br />K = 30 mA<br />Threshold of let-go is unlike AC there is no definable threshold of let-go for DC for<br />current magnitude below approximately 300 mA. Only the making and breaking of<br />current leads to painful and cramp like contractions of muscles.<br />Above approximately 300 mA, let-go may be impossible or only possible after<br />several seconds or minutes of shock duration. Below approximately 300 mA a<br />sensation of warmth is felt in the extremities during the flow of current. Above 300<br />mA unconsciousness frequently occurs.<br />36. Why AC is more dangerous than DC?<br />Because the excitatory action of current (stimulation of nerves and muscle, induction<br />of cardiac atrial or ventricular fibrillation) are linked to the changes of current<br />magnitude especially when making and breaking of the current. To produce the same<br />excitatory effects the magnitude of direct current flow of constant strength in 2 to 4<br />times greater than that of alternating current.<br />Question and answers Electrical Maintenance Unit<br />- 161 -<br />Transformer oil tests<br />1. What are the characteristics of transformer oil?<br />Characteristics Requirement Method of testing Remarks<br />Appearance The oil shall be clear<br />and transparent and<br />free from suspended<br />matter of sediments<br />A representative sample<br />of the oil shall be<br />examined in a 100-mm<br />thick layer at 27°C.<br />Density at 29.5°C<br />max.<br />0.89 gm / cm3 IS-1448(P:16):1977 See note 1<br />Kinematic viscosity<br />max. at<br />a. 27°C<br />b. 40°C<br />27 cSt<br />under consideration<br />IS-1448(P:25):1976<br />Interfacial tension at<br />27°C minimum<br />0.04 N/m IS- 6104:1971<br />Flash point penskymarten<br />(closed)<br />minimum<br />140°C IS-1448(P:21):1970<br />Pour point max. -6°C IS-1448(P:10):1970<br />Neutralization value<br />a. Total acidity max<br />b. Inorganic<br />acidity/alkalinity<br />0.03 mg KOH/g<br />nil<br />IS-1448(P:2):1967<br />IS-1448(P:2):1967<br />Alcoholic<br />potassium<br />hydroxide<br />solution of<br />0.02 N<br />should be<br />in place of<br />0.1 N<br />indicated<br />in test<br />method<br />Corrosive sulphur Non-corrosive<br />Electric strength<br />(Breakdown voltage)<br />a. New unfiltered<br />oil minimum.<br />b. After filtration<br />minimum<br />30 kV (rms)<br />If the above value is<br />not attained the oil<br />shall be filtered 60 kV.<br />IS-6792:1972<br />See note 2<br />Dielectric dissipation<br />factor (tan δ) at 90°C<br />max.<br />0.002 IS-6262:1971 See note 2<br />Question and answers Electrical Maintenance Unit<br />- 162 -<br />Specific resistance<br />(resistivity)<br />a. At 90°C min.<br />b. At 27°C min.<br />35 * 1012 Ω-cm<br />1500* 1012 Ω-cm<br />IS-6103:1971 See note 2<br />Oxidation stability<br />a. Neutralization<br />value after<br />oxidation max.<br />b. Total sludge,<br />after oxidation<br />max.<br />0.4 mg KOH/g<br />0.1% by weight.<br />Ageing<br />characteristics after<br />accelerated ageing<br />(open beaker method<br />with copper catalyst)<br />a. Specific resistance<br />at 27°C minimum &amp;<br />at 90°C minimum.<br />b. Tan δ at 90°C<br />max.<br />c. Total acidity max<br />d. Total sludge max.<br />2.5 * 1012Ω-cm<br />0.2 * 1012Ω-cm<br />0.20<br />0.05 mg KOH/g<br />0.05% by weight.<br />IS-12177:1987<br />IS-6103:1971<br />IS-6262:1971<br />IS-1448(P:2):1967<br />IS-12177<br />Presence oxidation<br />inhibitor<br />The oil shall contain<br />antioxidant additives<br />IS-13631:1992 See note 3<br />Water content max. 50 ppm IS-13567:1992<br />SK value Under consideration<br />Notes:<br />1. Density of the oil may be measured at ambient temperature and converted to 29.5°C<br />using the following equation.<br />29.5ρ = ρt {1+X (t-29.5)}<br />Where t = Ambient temperature (in °C)<br />ρt = Density measured at temperature t<br />X = Correction factor (Equal to 65 * 10-5).<br />2. As a consequence of the tendency for water absorption to occur due to breathing on<br />storage even when drums are sealed the oil shall be filtered to remove moisture and<br />particulate contaminates present in the original sample before the test as follows.<br />a. A sufficient quantity of oil is heated to 90 ± 2°C, then filtered hot under vacuum<br />corresponding to an absolute pressure of about 2.5 kPa through a sintered glass<br />filter of porosity grade 4’.<br />Question and answers Electrical Maintenance Unit<br />- 163 -<br />b. A portion of filtered is cooled in a desiccator and used immediately to measure<br />electric strength, if required, and specific resistance at 27°C. The remaining hot<br />filtrate is immediately used for measuring dielectric dissipation factor at 90°C and<br />specific resistance at 90°C.<br />3. For both phenol and amine types of indicators, qualitative methods shall be adopted.<br />In case of ambiguity (marginal cases) in finding the intensity of colour, a quantitative<br />method shall be adopted. Value of 0.5 (max.) shall be treated as absence of DBPCPhenolic<br />type inhibitor (quantitative method for amine is under consideration).<br />Question and answers Electrical Maintenance Unit<br />- 164 -<br />2. What are the permissible limits for the transformer oil?<br />Test<br />required<br />Equipment<br />voltage<br />Permissible<br />limits<br />Importance<br />Electric<br />strength<br />(breakdown<br />voltage)<br />min.<br />Above 145 kV<br />145 – 72.5 kV<br />50 kV<br />40 kV<br />30 kV<br />The electric strength does not give a<br />true indication of the deteriorated<br />condition of the oil. An oil which is<br />significantly oxidised under high<br />temperature may show a high dielectric<br />strength in the absence of moisture. The<br />presence of oil deterioration particles,<br />water and foreign contaminants results<br />in general overall reduction in the<br />efficiency of the equipment. A normal<br />method of oil filteration and<br />dehydration only maintain the electric<br />strength but does not improve the<br />deteriorated oil. It is therefore not<br />advisable to rely solely on the electric<br />strength of the oil by periodic tests<br />without verifying its other<br />characteristics.<br />Water<br />content<br />(max.)<br />Above 145 kV<br />Below 145 kV<br />25 PPM<br />35 PPM<br />The presence of water in oils is harmful<br />as it lowers the electric strength and<br />resistivity. And it reacts with solid<br />insulating materials particularly paper.<br />Dielectric<br />dissipation<br />factor (Tan<br />δ delta) at<br />90°C max.<br />Above 145 kV<br />Below 145 kV<br />0.2 max.<br />1.0 max.<br />This characteristic is very sensitive to<br />the presence in the oil of soluble<br />contaminants and ageing products. This<br />test is therefore of special interest. If tan<br />delta increases resistivity decreases.<br />This is highly influenced by<br />temperature, voltage, and frequency of<br />the equipment.<br />Resistivity<br />(min) 90°C<br />All voltages 0.1 * 1012<br />Ω - cm.<br />The specific resistance is another<br />important test for the quality of oil.<br />High resistivity reflect low contents of<br />free ions and ion forming particles and<br />normally indicates low concentration of<br />conducting contaminants. Water<br />contents and cold precipitable materials<br />can reduce the resistivity.<br />Neutralizati All voltages 0.5 mg The acid products formed by the<br />Question and answers Electrical Maintenance Unit<br />- 165 -<br />-on value<br />(max.)<br />KOH/g oxidation of the oil activity encourage<br />deterioration of insulating paper and<br />pressboard. It is therefore essential to<br />detect and monitor this process. The test<br />is required to be performed more<br />frequently if value exceeds 0.3 mg<br />KOH/g.<br />Sediment<br />and<br />precipitable<br />sludge<br />All voltages No<br />sediments<br />of sludge<br />should be<br />detectable<br />The presence of those particle normally<br />reduces the electric strength of the oil<br />and in addition deposits hinder heat<br />exchange, thus encouraging from the<br />deterioration of the insulating material.<br />Flash point All voltages 125°C The test is for finding lower<br />hydrocarbons which formed due to<br />some incipient fault in the equipment<br />such as electrical discharge, excessively<br />high internal temperature core fault etc.<br />this test should be made more<br />frequently if the oil has been subjected<br />to high temperature or shows any sign<br />of unusual odour.<br />Interfacial<br />tension at<br />27°C (min.)<br />All voltages 0.018 N/m The interfacial value of oil against<br />water provided a very sensitive means<br />of determining the degree of oil<br />contamination. We can measure the<br />concentration. A low interfacial value<br />indicates that the oil is damaged.<br />Dissolved<br />gases<br />(max.) PPM<br />All voltages IS :10593<br />1983<br />Under normal service conditions only<br />small amount of CO, CO2 and very<br />small quantity of H2 and hydrocarbons<br />are found. Large amount of these gases<br />is an indication of an incipient due to<br />overheating, sparking, hotspot, arcing,<br />selector breaking current, solid<br />insulation deterioration etc.<br />Knowledge of the effect of such faults<br />in operation and safety of the power<br />apparatus is of great importance, as<br />transformers are required to opesrate<br />over a long period of time.<br />To obtain such information and rectify<br />the faults at regular intervals (using<br />Question and answers Electrical Maintenance Unit<br />- 166 -<br />dissolved gas analysis method) ensures<br />trouble free operation and safety of<br />equipment.<br />Question and answers Electrical Maintenance Unit<br />- 167 -<br />Recommended or permissible values for 220 kV switchyard.<br />1. Rated voltage of equipment = 220 (Nominal system voltage) * 1.1<br />= 245 kV<br />2. Permissible duration of short circuit in network 220 kV nominal voltage = 180 ms.<br />3. Permissible over voltage factors for 220 kV nominal voltage system<br />a. = 220 * 6.5<br />3<br />= 825 kV (Approx.)<br />b. Power frequency flash over (wet) voltage<br />= 220 * 3.0<br />3<br />= 380 kV (Approx.)<br />4. Cable charging breaking current requirement:-<br />The CB for opening high voltage cable or cable networks should be capable of<br />interrupting the charging currents of cables successfully with the over voltage within<br />specified limits. The recommended value of rated cable charging breaking current<br />for 220 kV (Nominal voltage) is 250 A.<br />5. Impulse and power frequency with stand levels for various system voltages<br />(Applicable at 20°C 760 mm of Hg pressure and 11-g/m3 humidity).<br />Impulse withstand kV<br />crest<br />One minute power<br />frequency test voltage kV<br />(rms)<br />Nominal<br />system<br />voltage<br />(L-L) kV<br />(rms)<br />Highest<br />Rated voltage<br />(L-L) kV<br />(rms) Full<br />insulation<br />(kV)<br />Reduced<br />insulation<br />(kV)<br />Full<br />insulation<br />(kV)<br />Reduced<br />insulation<br />(kV)<br />220 245 1050 900 460 395<br />Note: Reduced insulation value – applies where internal insulation is mire important.<br />Full insulation value – applies where external insulation is more important.<br />6. Standard clearances:-<br />For rated nominal system voltages of the order of 220 kV<br />Minimum clearance to earth = 117.8 cms.<br />Minimum clearance between phase in air = 205.8 cms.<br />Note: clearances indicated above are applicable for effectively earthed system.<br />7. Duty cycle for 220 kV ABCB:<br />Question and answers Electrical Maintenance Unit<br />- 168 -<br />0 – 3” – CO – 3” – CO.<br />Question and answers Electrical Maintenance Unit<br />- 169 -<br />8. Operating time for 220 kV breaker:<br />Opening time - 19 to 23 m sec.<br />Closing time - 45 to 54 m sec.<br />Blast time - 26 to 40 m sec.<br />9. mV drop across the arc chambers:<br />Acceptance limit – 35 mV.<br />10. 220 kV isolators:<br />a. mV drop test for the main contact – 11 mV for 1250 A isolators.<br />7.5 mV for 2000 A isolators.<br />b. Interrupting capacity of magnetising current – 0.8 A at 0.15 PF (lag)<br />c. Interrupting capacity for line charging current – can interrupt charging currents of<br />bus bars and cables of upto 20 – 220 kV bays.<br />11. 220 kV transformers:<br />Arcing horns settings for 220 kV transformer having BIL = 900 kV should be 1200<br />mm.<br />Question and answers Electrical Maintenance Unit<br />- 170 -<br />Electrical or electronic equipment design factors<br />1. Explosion protection<br />A source of energy along with the concentration of the following factors in the<br />atmosphere is all that required to trigger off an Explosion in hazardous locations.<br />a. Flammable substances such as gas, vapour, mist and dust.<br />b. Air / Oxygen present in the atmosphere.<br />c. Ignition level.<br />The factors leading to explosion also depends upon the inherent properties of gas and<br />its concentration in the atmosphere.<br />Developing and designing of electrical or electronic products for explosion<br />protection is very much vital for safety purpose of human life as well as for plant<br />sites. For design and selection of an equipment for hazardous area, it is very much<br />essential to know the parameters or characteristics of the atmosphere. Measures have<br />to be taken to prevent formation of explosive atmosphere and restricting the<br />explosion to a safe level. Those hazardous locations are classified in to zones and<br />areas as per NEC and IEC classifications.<br />Types of protection.<br />Areas where explosive atmospheres can occur despite the explosion protection<br />measures employed, only explosion protected electrical equipment may be used.<br />Explosion protected electrical equipment can be manufactured to following<br />protection type levels.<br />Protection<br />type<br />Basic principal Principal application<br />Flame –<br />proof<br />enclosure d<br />Part which can ignite an explosive<br />atmosphere are placed in an enclosure<br />which, if there is an ignition of an<br />explosive mixture internally, will<br />withstand the pressure and prevent the<br />explosion being transmitted to the<br />atmosphere around the enclosure.<br />Switchgear and switching<br />installations, control and<br />display units, control<br />boards, motors,<br />transformers, heating<br />devices, light fittings.<br />Increased<br />safety e<br />Additional measures are taken to<br />achieve a higher level of safety and<br />avoid the risk of impermissibly high<br />temperature and the occurrence of<br />electrical equipment, which in normal<br />use produce neither sparks arcs or<br />dangerous temperature.<br />Terminal and connected<br />boxes, control boxes for<br />the installation of<br />excomponents (which are<br />protected in another<br />protection call), squirrel<br />cage motors, light fittings.<br />Pressurised<br />apparatus p<br />The formation of an explosive<br />atmosphere inside an enclosure is<br />prevented by using a protective gas to<br />maintain an internal overpressure<br />Switching and control<br />cabinets, analysis devices,<br />large motors.<br />Question and answers Electrical Maintenance Unit<br />- 171 -<br />relative to the surrounding atmosphere,<br />and if necessary, the interior of the<br />enclosure is permanently supplied with<br />protective gas so that there is dilution of<br />flammable mixtures.<br />Intrinsic<br />safety i<br />The equipment placed in the hazardous<br />are contains only intrinsically safe<br />circuits. A circuit is intrinsically safe if<br />no sparks or thermal effects occur under<br />established test conditions (including<br />the normal operating and certain fault<br />conditions), which could lead to the<br />ignition of a given explosive<br />atmosphere.<br />Measurement and control<br />equipment,<br />communications<br />equipments, sensors,<br />actuators.<br />Oil<br />immersion<br />(o)<br />Electrical equipment or parts of<br />electrical equipment are immersed in a<br />protective liquid in such a way that an<br />explosive atmosphere above the surface<br />or outside the enclosure cannot be<br />ignited.<br />Transformers, starting<br />resistors.<br />Powder<br />filling q<br />Type of protection by which the<br />equipment parts that could become and<br />ignition source are fixed in position and<br />completely surrounded by finely ground<br />solids, so as to prevent ignition of an<br />external explosive atmosphere.<br />Electronic devices<br />Moulding m Parts, which can ignite an explosive<br />atmosphere, are embedded in a casing<br />compound so that the explosive<br />atmosphere cannot be ignited.<br />Switchgear for low<br />powers, control gear and<br />indicating equipment,<br />display equipments,<br />sensors.<br />Question and answers Electrical Maintenance Unit<br />- 172 -<br />2. Index of protection (IP)<br />IP (index of protection) for enclosures of electrical equipment as per IS: 13947<br />(Part-1): 1993 are as following.<br />Protection against solids Protection against liquids Mechanical protection<br />IP Principal IP Principal IP Principal<br />0 No protection. 0 No protection. 0 No protection<br />1 Protected against solid<br />bodies larger than 50<br />mm (eg:- accidental<br />contact with the hand).<br />1 Protected against<br />vertically falling<br />drops of water<br />(condensation).<br />1 Impact energy 0.225<br />joule.<br />2 Protected against solid<br />bodies larger than 12<br />mm (eg:- finger of the<br />hand).<br />2 Protected against<br />drops of water<br />falling at upto 15°<br />from the vertical.<br />2 Impact energy 0.375<br />joule.<br />3 Protected against solid<br />bodies larger than 2.5<br />mm (eg:- tools, wires).<br />3 Protected against<br />drops of rain water<br />at upto 60° from the<br />vertical.<br />3 Impact energy 0.500<br />joule.<br />4 Protected against solid<br />bodies larger than 1<br />mm (fine tools and<br />small wires).<br />4 Protected against<br />projections of water<br />from all directions.<br />5 Impact energy 2.00<br />joule.<br />5 Protected against dust<br />(no harmful deposit).<br />5 Protected against jets<br />of water from all<br />directions.<br />7 Impact energy 6.00<br />joule.<br />6 Completely protected<br />against dust.<br />6 Protected against jets<br />of water of similar<br />force to heavy seas.<br />9 Impact energy 20.00<br />joule.<br />7 Protected against the<br />effects of immersion.<br />8 Protected against<br />prolonged effects of<br />immersion under pressure.<br />Question and answers Electrical Maintenance Unit<br />- 173 -<br />Question and answers Electrical Maintenance Unit<br />- 174 -<br />Thyristor engineering<br />Introduction<br />Thyristor is the name of a large family of semiconductor devices, which includes the<br />following.<br />a. Silicon controlled rectifier (SCR).<br />b. Triac.<br />c. Diac.<br />d. Silicon controlled switch (SCS).<br />e. Light activated switch (LAS) etc.<br />But in general the silicon controlled rectifier is referred to as thyristor. This device finds<br />extensive applications in industrial equipments such as rectifiers, inverters, choppers<br />etc. In our station thyristors are used in the following equipments.<br />a. Main generator static excitation system.<br />b. Power UPS.<br />c. Control UPS.<br />d. Diesel generator excitation system and etc.<br />Construction of thyristor<br />The thyristor is a four-layer P-N-P-N semiconductor device. The biasing at the three<br />junctions J1, J2, J3 determine the state of the thyristor. Ohmic connections are made to<br />the P, P, N regions and these terminals thus formed are called Anode, Gate, and<br />Cathode respectively. This is shown in the figure below.<br />J1 J2 J3<br />A C<br />A P N P N C<br />G<br />G<br />Difference between diode and thyristor<br />Diode is an uncontrolled rectifier device whereas a thyristor is a controlled rectifier<br />device. The condition for the conduction of a diode is that the anode must be positive<br />with respect to the cathode. In case of a thyristor in addition to the above condition a<br />positive gate pulse should also be applied to the gate terminal. By controlling the instant<br />of the pulse release the conduction of the thyristor can be controlled.<br />A C<br />AC input DC output<br />Question and answers Electrical Maintenance Unit<br />- 175 -<br />V – I characteristics of thyristor<br />The thyristor characteristics are divided into four regions of operation. They are as<br />follows.<br />a. Forward blocking region.<br />b. Forward conduction region (Useful region of operation).<br />c. Reverse blocking region.<br />d. Reverse conduction or breakdown region.<br />a. Forward blocking region: When an external voltage is applied to the thyristor<br />making anode positive with respect to the cathode, the thyristor is said to be ‘forward<br />biased’. In this conditions<br />1. Junctions J1 &amp; J3 are forward biased.<br />2. Junction J2 is reverse biased.<br />3. A small forward leakage current flows which increases with the applied voltage.<br />The thyristor is in the ‘off state’ since the voltage applied is less than the break over<br />voltage of the device. This is represented by region OA in the characteristic graph.<br />b. Forward conduction region: As the forward voltage is increased, a point is reached<br />where the junction J2 gets forward biased and allows a large current to flow through<br />the device. This voltage is known as the ‘forward break over voltage’. Above this<br />point the voltage across the device falls to a low value and the current is limited only<br />by the external load resistance. This is represented ‘purpose of gate triggering’.<br />As seen above thyristors can conduct even in the absence of gate pulses provided the<br />forward voltage across them is more than the break over voltage. The application of<br />the positive gate pulse reduces the break over voltage and the thyristor starts<br />conducting at a much lower forward voltage. This characteristics of the thyristor<br />makes it possible to control its conducting period in each cycle of the applied voltage<br />by the release of gate pulses at the desired instant. The firing circuit or the pulse<br />generator generates the firing pulses, the position (with reference to the voltage<br />across the thyristor) of which depends on the DC voltage signal given to it by the<br />controller (voltage, current regulator). This is shown in the diagram given below.<br />AC Input<br />Synchronizing Voltage<br />Thyristor<br />V ref<br />Controller Pulse Gen. Pulse<br />V feed back (AVR) (Firing ckt) amplifier<br />Question and answers Electrical Maintenance Unit<br />- 176 -<br />Question and answers Electrical Maintenance Unit<br />- 177 -<br />c. Reverse blocking region: When a reverse voltage is applied across the thyristor in<br />such a way that the anode is at a negative potential with respect to the cathode the<br />thyristor is said to be ‘reversed biased’. Under this condition,<br />1. Junctions J1 and J3 are reversed biased.<br />2. Junction J2 is forward biased.<br />Only a small leakage current flows through the device, which increases with the<br />applied voltage. This is indicated by region OD in the characteristics.<br />d. Reverse conduction region: When the reverse voltage across the thyristor is<br />increased a point is reached when the junctions J1 and J3 breakdown causing heavy<br />current to flow through the device. The voltage at this point is known as the ‘reverse<br />breakdown voltage’. This is indicated by region DE in the characteristics.<br />Current C<br />Forward conduction region<br />IL<br />IH<br />A<br />B IG1 IG=0<br />O<br />D VBO Voltage<br />Reverse blocking<br />region Forward blocking region<br />Reverse conduction<br />region<br />IL – latching current.<br />IH – holding current.<br />VBO – break over voltage.<br />IG – gate current.<br />Question and answers Electrical Maintenance Unit<br />- 178 -<br />Some important technical terms<br />a. Latching current: It is the minimum ON state current required to keep the thyristor in<br />the ON state after the triggering pulse has been removed. In control and power<br />ACVR’s this has been achieved by bleeder or dummy load resistances connected<br />across the output terminals (DC side).<br />b. Holding current: It is the value of anode current below, which the thyristor in<br />conduction (ON state) turns OFF. Thus holding current is ON state to OFF state<br />current where as latching current is OFF state to ON state current.<br />c. Firing angle: The instant at which the gate pulse is released expressed in electrical<br />degrees with reference to the applied voltage across the thyristor is known as ‘firing<br />angle’. For rectifier mode of operation the firing angle will be between 0° to 90°.<br />Greater the firing angle lesser will be the output voltage of the rectifier. This is<br />illustrated in the figure given below.<br />Firing angle 30° Firing angle 70°<br />0° 180° 360° 0° 180° 360°<br />30° 70°<br />VDC VDC<br />V VDC V VDC<br />d. Triggering: The process of switching the thyristor ON by the application of the gate<br />pulse is known as triggering.<br />e. Ripple: The AC components in the DC output of any rectifier are called the ripple. In<br />control and power ACVR’s on no load the ripple voltage is about 80 V AC at a DC<br />output voltage of 260 V. This AC voltage indicates the conduction of all the<br />thyristors in the bridge. For example the ripple voltage of ACVR’s rise to about 140<br />V AC if one thyristor of the bridge does not conduct.<br />f. Filter: Filters are used to remove the ripple components from the output of any<br />rectifier so that it does not reach the load circuit. Inductors and capacitors are used as<br />filters in the output of rectifiers.<br />Question and answers Electrical Maintenance Unit<br />- 179 -<br />Question and answers Electrical Maintenance Unit<br />- 180 -<br />Protection of thyristors<br />The thyristor is a very sensitive semiconductor device and it needs to be protected for<br />the following abnormal conditions while in service.<br />a. High dv/dt.<br />b. High di/dt<br />c. Short circuit / over current.<br />High dv/dt: This indicates the rate of rise of anode voltage. This rating specified for a<br />particular thyristor should not be exceeded because it would lead to spurious triggering<br />(switching ON) of the thyristor. The ‘snubber circuit’ (a resistance and a capacitance in<br />series) connected across the thyristor as shown below provides the protection against<br />high dv/dt.<br />R C<br />Thyristor<br />High di/dt: This rating of the thyristor indicates the maximum rate of rise of ON-state<br />current. When a thyristor is turned ON conduction starts at one or more places near the<br />gate. Small area of conduction then spreads from these points to the whole crystal.<br />Sudden rise of current causes ‘hot spots’ in the junctions and subsequent failure of the<br />device due to melting. Connecting an inductor in series with the thyristor shown below<br />provides protection against high di/dt.<br />R C<br />L<br />Thyristor<br />Short circuit protection: A semi conductor fuse in series with the thyristor provides<br />protection against short circuits. The semi conductor fuses operate very fast with prearcing<br />time less than 0.5 m-sec and arcing time of about 3 m-sec. Hence the fault<br />current will be interrupted by these fuses before it reaches its maximum value.<br />R C Isc<br />L S.C. Fuse<br />Thyristor<br />Arcing time<br />Clearing time<br />Melting time<br />Semi conductor fuse characteristics<br />Question and answers Electrical Maintenance Unit<br />- 181 -<br />Testing of thyristor<br />1. Resistance checks: The anode – cathode resistance and gate – cathode resistance of<br />the thyristor should be as follows.<br />Anode – cathode resistance for power thyristors<br />In the forward direction – about 1 MΩ.<br />In the reverse direction – about 1 MΩ.<br />Gate – cathode resistance about 25Ω in both the directions.<br />In case of fused thyristor these resistances will be zero Ω.<br />2. Current deflection test: The thyristor should be connected to a power supply as<br />shown below.<br />A K Ammeter<br />G<br />R Switch<br />Power supply<br />The moment the switch is closed the thyristor conducts and the ammeter reads the<br />current. If there is no deflection in the meter it shows that the thyristor is faulty. This<br />is a foolproof method for testing any thyristor. The above testing can also be done<br />with the help of a motwane analog multimeter. The arrangement for the same is as<br />follows.<br />A K<br />G<br />(+)<br />Ammeter<br />Switch<br />(–)<br />Motwane ammeter selected in resistance range.<br />Switch open high resistance.<br />Switch closed zero resistance.<br />Question and answers Electrical Maintenance Unit<br />- 182 -<br />Question and answers Electrical Maintenance Unit<br />- 183 -<br />Miscellaneous (Tests on power cables)<br />1. What are precautions to be taken while doing maintenance or repair work on power<br />cables?<br />A research organised by EPRI (electric power research institute) on medium voltage<br />XLPE cables found that DC high potential at 80% of the factory value<br />a. Subsequently reduces the life of the cable and<br />b. It did not identify significantly weakened cable.<br />Based on above research recommended maintenance proof test voltage = 60% of the<br />factory test voltage.<br />Maintenance and repair:<br />Before attempting for any corrective maintenance on power cables like replacement<br />of lugs or jointing of cables following should be taken care<br />a. IR value should be good and it should be comparable to the previous values.<br />b. The quality of joints should be such that it dies not add any resistance to the<br />circuit. Before jointing is commenced it is advisable that IR of both sections of<br />cable to be jointed be checked.<br />c. Before jointing a paper insulating cable (for PVC cables this step is not required),<br />the paper insulation should be tested for the presence of moisture by immersion in<br />hot compound for paraffin wax at a temperature between 120°C and 140°C. the<br />presence of moisture indicated by the formation of bubbles when a piece of the<br />paper is immersed in hot compound. Use only single strip of the paper.<br />d. In case dia of die to be used for crimping is slightly more than dia of cables, then<br />use some loose strand before doing crimping of lug. While crimping it should be<br />ensured that homogeneity of cramped conductor strands is achieved otherwise it<br />will add resistance and create over heating.<br />2. What are the tests to be carried out during DC high voltage test on power cables?<br />DC high voltage testing.<br />During DC high voltage testing flow of following currents will take place.<br />a. Capacitance charging current.<br />b. Dielectric absorption current.<br />c. Surface leakage current.<br />d. Partial discharge current (corona).<br />e. Volumetric leakage current.<br />a. Capacitance charging current.<br />The capacitance charging current is high as the DC high potential is applied and can<br />be calculated by the formula<br />ig = E – t where ig – capacitance charging current.<br />rc / R E – voltage in kilovolts.<br />r – resistance in mega ohms.<br />c – capacitance in micro farads.<br />Question and answers Electrical Maintenance Unit<br />- 184 -<br />t – time in seconds.<br />The charging current is a function of time and will decrease as the time of the<br />application of voltage increases. It is the initial charging current when voltage is<br />applied and therefore not of any value for test evaluation. Test readings should be<br />taken until this current has decreased to a sufficiently low value.<br />Question and answers Electrical Maintenance Unit<br />- 185 -<br />b. Dielectric absorption current.<br />The Dielectric absorption current is also high as the test voltage is applied and<br />decreases as the voltage applicable time increases. This current can be calculated by<br />the formula<br />ia =VCDT-n<br />Where ia – dielectric absorption current.<br />V – test voltage in kilovolts.<br />C – capacitance in micro farads.<br />D – proportionately constant.<br />T – time in seconds.<br />n – constant.<br />Again time should be allowed before recording test readings so that this current has<br />decreased sufficiently.<br />c. Surface leakage.<br />The surface leakage current is due to the conduction on the surface of the insulation<br />and not desired in test results and should therefore be eliminated by carefully<br />cleaning the surface.<br />d. Partial discharge current.<br />The partial discharge current, also known as corona current is caused b ionization of<br />air due to high-test voltage. This current is not desirable and same is normally<br />controlled by providing semi-conducting tape to separate the conductor from<br />insulation.<br />Semi-conducting tape is used to separate the conductor from the insulation to prevent<br />possible damage of the insulation from the corona and ionization. The voltage may<br />develop between stranded conductor and insulation, thereby causing the ionization of<br />air and breakdown of cable insulation. The application of semi-conducting smoothes<br />the voltage stress and keeps such voltage stress constant and to a minimum.<br />e. Volumetric leakage current.<br />The volumetric leakage current flows through the insulation volume itself. This is the<br />current that is used to evaluate the condition of the insulation under test. Sufficient<br />time should be allowed for the volumetric current to stabilize before test readings are<br />recorded.<br />Question and answers Electrical Maintenance Unit<br />- 186 -<br />Predictive (preventive) maintenance on Induction Motor.<br />1. What are the reasons for high current in motor?<br />a. High frequency – at 51 Hz current will be 105% of the normal current.<br />b. Low frequency – at 47.8 Hz current will be 102% of the normal current.<br />c. High voltage.<br />d. Under voltage.<br />e. Mechanical over loading.<br />f. Process requirement.<br />2. What are the reasons for unbalanced current in motor?<br />a. Loose power cable connection.<br />b. Voltage unbalance.<br />c. Short-circuited turns of coils of winding.<br />3. What are the reasons for vibration in the motor?<br />Vibration could be because of mechanical faults and electrical faults.<br />1. Mechanical faults.<br />a. Wrong alignment of the motor on foundation.<br />b. Wrong installation.<br />c. Improper fitting of bearing and cooling fans.<br />d. Periodic impulse loads such as reciprocating compressors.<br />e. Pulley of heavy weight which cause bending of motor shaft resulting in non<br />uniform air gap.<br />f. Damage of bearing or bad bearing.<br />g. Bad coupling.<br />h. If the axial alignment of the motor and the driven machine is incorrect and<br />rotor is allowed to contact its axial stops, high axial vibrations may occur,<br />together with high bearing temperature high and even bearing failure.<br />i. Machine base and foundation problem.<br />j. Under sized bearing.<br />Question and answers Electrical Maintenance Unit<br />- 187 -<br />2. Electrical faults.<br />a. Air gas dissymetry.<br />b. Broken rotor bars.<br />c. Slackened stator core.<br />d. Slackened rotor core.<br />e. Interturn short in the rotor winding in the two-pole machine.<br />f. Unbalance in rotor winding.<br />g. Unbalance power supply voltages.<br />If the vibration is because of electrical fault, de-energise the machine and<br />watch the vibration as it runs down.<br />The possible vibration frequencies observed are<br />a. Twice the power supply frequency – it indicates that the vibration is developed<br />by unbalanced power supply voltages, unbalanced air gap, unbalance in rotor<br />winding, slackened stator core etc.<br />b. Multiple of power frequency – the stator and rotor slots co-ordinate to develop<br />radial lines of force to deform and pulsate the cores.<br />c. Twice the slip frequency – magnetic unbalance due to unbalance air gaps,<br />slackened rotor core, interturn short in the rotor-winding etc. of two-pole<br />machine.<br />d. Beat (Humming) – in case of two-pole machine the beat is developed when<br />the vibration of twice as much as power frequency developed between the<br />stator and rotor is superimposed on the vibration of twice the slip frequency<br />developed due to irregular air gap.<br />4. What are the reasons for winding temperature high in the motor?<br />For motors having class – B insulation the temperature should not be more than<br />110°C and for motors having class – F insulation the temperature should not be more<br />than 130°C. In case temperature is more, then the following could be the possible<br />reasons.<br />1. Electrical overloads.<br />a. Over and under voltage.<br />b. Over and under frequency.<br />c. Voltage unbalance. Voltage unbalance create unbalance of currents and<br />increase in temperature which will be 2*(% voltage unbalance)* (% voltage<br />unbalance)*.<br />(% Voltage unbalance) = 100 * maximum deviation from average voltage<br />average voltage.<br />For instance if voltages are 390V, 410V &amp; 440V,<br />% Voltage variation = 100*(440-390+410+440) (440-390+410+440) = 6.45%.<br />3 3<br />Therefore increase in temperature rise = 2*(6.45)*(6.45) = 83°C (approximately).<br />d. Voltage transients and interruptions.<br />Question and answers Electrical Maintenance Unit<br />- 188 -<br />e. Loose connection at motor terminals.<br />f. Unbalance current.<br />g. Single phasing (if OLR protection is not working).<br />h. Long acceleration cycle.<br />i. Unusual system grounding conditions.<br />Question and answers Electrical Maintenance Unit<br />- 189 -<br />2. Mechanical overloads.<br />a. Locked rotor.<br />b. Heavy starting.<br />c. Bearing problem.<br />d. Overload in continuous duty and intermittent duty.<br />3. Environmental overloads.<br />a. Excessive temperature of cooling medium or ambient temperature.<br />b. Restricted flow of cooling.<br />c. Reduction in the density of cooling medium.<br />d. Heat transfer from machine parts connected to the motor.<br />4. Others.<br />a. Excessive number of switching operations.<br />5. What are the reasons for bearing temperature high?<br />Temperature of bearing should not be more than 90°C. In case temperature is higher<br />than the 90°C the following could be the possible reasons.<br />a. Inadequate lubricants inside the bearing.<br />b. Faulty bearing.<br />c. Bearing is jammed.<br />d. Over greasing.<br />e. Improper grade of lubricant.<br />6. What are the reasons for abnormal sound or noise?<br />Motors in general should run very quietly and no abnormal noise is desired.<br />However if noise is there, it could be because of following reasons.<br />a. Windage noise – the noise due to ventilating system, (whistling noise).<br />b. Bearing noise – the noise due to its rolling contact.<br />c. Unusual noise – some defects inside the motor (example – motor bar failure).<br />d. Deep heavy growling noises – some electrical fault.<br />For permissible limits of noise levels for rotating electrical machines IS: 12065:1987<br />is being reffered.<br />7. What are the reasons for harmonics in the motor?<br />Generally even harmonics are not expected to be present in three phase motors.<br />Triple-n harmonics like 3rd, 9th, 15th etc. are also not expected. The dominant odd<br />harmonics expected are 5th, 7th, 11th and 13th etc.<br />Presence of strong 2nd harmonics indicates unbalance voltage, unbalance winding<br />impedance, rotor defects, magnetic imbalance, faulty rotor skewing etc.<br />Very strong 3rd harmonics indicates magnetic saturation, ground leak currents,<br />overloads etc. Overloading causes overheating, resulting in non-linear magnetization<br />Question and answers Electrical Maintenance Unit<br />- 190 -<br />which gives high 3rd harmonic winding faults, short circuits. Hot spots in rotor or<br />stator also may indicate higher harmonics.<br />Question and answers Electrical Maintenance Unit<br />- 191 -<br />8. What are the possible reasons for not coming of rated speed during start?<br />In case motor does not come to its rated speed then following could be the probable<br />causes.<br />a. Starting load is too high.<br />b. Broken rotor bars (look for cracks near rings).<br />c. Open primary circuit.<br />d. Voltage is too low.<br />9. What are the possible reasons for motor to take long acceleration time?<br />Following may the possible reasons for motor to take long acceleration time.<br />a. Excess loading.<br />b. May be rewound motor with poor quality of winding conductor having high<br />resistance.<br />c. Defective squirrel cage rotor.<br />d. Applied voltage is too low.<br />10. What are the points contributes in insulation resistance of the motor?<br />If the measured insulation resistance of the motor is less than 1 MΩ / kV with a<br />minimum of 1MΩ, when the machine is cold it is to be dried out before full voltage<br />is applied to the terminals of the motors and the drying out is to be continued as long<br />as the insulation resistance rises or until a sufficiently high value that is not less than<br />1 MΩ / kV with minimum of I MΩ at 75°C is reached.<br />While proceeding for point as above said, following factors are to be kept in mind<br />which affect the insulation resistance measurement. They are,<br />a. Surface condition.<br />b. Moisture.<br />c. Temperature.<br />d. Magnitude of test voltage.<br />e. Duration of application of test voltage.<br />f. Residual charge in the winding.<br />g. Ageing of the insulation.<br />h. Mechanical stresses.<br />11. What are the minimum recommended PI values for AC and DC rotating machines?<br />Following minimum recommended PI values criteria is to be followed.<br />a. 1.5 for class – A insulation.<br />b. 2.0 for class – B insulation.<br />c. 2.5 for class – F insulation.<br />Question and answers Electrical Maintenance Unit<br />- 192 -<br />12. What is the minimum recommended absorption coefficient value for AC and DC<br />rotating machines?<br />Absorption coefficient = IR value for 60 seconds = 1.3 (minimum recommended value)<br />IR value for 15 seconds<br />Tips:<br />a. IR value decreases some what with an increase I applied voltage. However for<br />machines in good condition substantially the same IR is obtained for any test<br />voltage up to the peak value of the rated operating voltage.<br />b. If the IR value decreases significantly with an increase in applied voltage it is an<br />indication of imperfections or fractures of the insulation aggravated by the<br />presence of dirt or moisture or may be due to the effects of dirt or moisture alone,<br />or may result from numerous other phenomena not necessarily associated with<br />any defect or weakness.<br />c. IR value for good dry winding continue to increase for hours with constant test<br />voltage continuously applied, however a fairly steady value is usually reached in<br />10 to 15 minutes. If the winding is wet or dry or dirty the steady value is usually<br />reached in 1 or 2 minutes after the test voltage is applied.<br />d. The recommended minimum IR value for AC and DC machines is determined by<br />the following empirical relationship.<br />IR = kV + 1<br />Where IR = recommended minimum IR in mega ohms at 40°C of the entire<br />machine winding and kV = rated machine voltage in kilo volts.<br />Temperature correction is to be applied, if winding is not at a temperature of<br />40°C.<br />e. IR of the one phase of three phases winding with other two phases earthed, is<br />approximately twice that of the entire winding. Therefore when the three phases<br />are tested separately, the observed insulation resistance of each phase should be<br />divided by two to obtain a value which after correction for temperature, may be<br />compared with the recommended minimum value of IR.<br />13. What is use of Tan – Delta test? And what are the recommended values?<br />The very purpose of this test is to detect moisture content, voids, cracks and<br />deterioration in the insulation and same is to be conducted on HT motors.<br />Based on the guidelines given in the article ‘Diagmistic testing on the winding<br />insulation’ by J. S. Simon (IEE vol. 127 may 1980) the contamination level of motor<br />winding is to be assessed from the given Tan – Delta value.<br />Starting Tan – Delta values Degree of contamination<br />0 – 4%. Low void content.<br />4 – 6%. Clean.<br />6 – 10%. Some dirt.<br />10 – 14%. Dirt and moisture.<br />14 – 16%. Gross contamination.<br />Question and answers Electrical Maintenance Unit<br />- 193 -<br />16 – 20%. Heavy deposit of oil dirt.<br />Above 20%. Severe oil and carbon contamination.<br />Question and answers Electrical Maintenance Unit<br />- 194 -<br />14. What are important guidelines for conducting HV test?<br />Based on the recommendations given in IS: 4029:1977 decided DC test voltage<br />= (2E+1kV) 1.6 * M<br />Where E = rated voltage.<br />2.6 = AC to DC conversion factor.<br />M = derator factor which is a function to be decided on the basis of the age and<br />condition of equipment.<br />The DC voltage applied in steps and the leakage current recorded at each step. A plot<br />leakage current Vs test voltage is to be plotted as the test progress.<br />Some recommendations of IS : 4029 : 1977.<br />a. The HV test made on the windings on acceptance shall as far as possible not be<br />repeated. If however a second test to be made at 80% of the voltage given by the<br />empirical formula given above.<br />b. Test voltage for completely rewound motor = full test voltage for new motor.<br />c. Partially rewound or overhauled motor = 75% * full test voltage for a new motor.<br />d. Before the test for the old parts of the winding shall be carefully cleaned and<br />dried.<br />e. Before attempting of HV DC test a minimum PI value of motor should be<br />obtained.<br />15. What are the uses of high voltage surge test?<br />This test gives distinct wave forms giving indications of various defects such as,<br />a. Turn to turn short in same phases.<br />b. Coil to coil short in same phases.<br />c. Partial phase to phase short.<br />d. Complete phase to phase short.<br />e. Improper coil connections.<br />f. Reverse coil connections.<br />g. Open coil connections.<br />h. Short to ground partial.<br />i. Short to ground complete.<br />Question and answers Electrical Maintenance Unit<br />- 195 -<br />16. What is the thumb rule for motor current?<br />Thumb rule for NO LOAD current of motors.<br />Type of enclosure No. of poles % No Load current of rated current<br />TEFC 2 15 – 20<br />TEFC 4 30 – 35<br />SPDP 2 25 – 30<br />SPDP 4 35 – 40<br />SPDP 6 to 8 50 – 55<br />SPDP 10 80<br />Note: TEFC (Totally enclosed fan cooled) motors are low inductive having low<br />active material in comparison to SPDP(Screen protected drip proof) motors.<br />Thumb rule for calculating positive sequence and negative sequence current in<br />motors.<br />a. Positive sequence current: Average of all three phases currents.<br />b. Negative sequence current: Maximum deviation of any of the phase currents from<br />the average.<br />17. How you evaluate the insulation condition based on PI value?<br />Evaluation of insulation condition based on PI value<br />PI value Insulation condition Recommendation<br />1.0 – 1.5 Bad Drying is mandatory<br />1.5 – 2.0 Doubtful Drying is recommended<br />2.0 – 3.0 Adequate No drying is needed<br />3.0 – 4.0 Good No drying is needed<br />> 4.0 Excellent No drying is needed<br />18. What are the conditions monitoring for the motor bearings?<br />Bearing oil analysis is a useful tool in determining bearing performance and possible<br />deterioration. Periodic checks for oil colour, viscosity and acidity can aid in<br />preventing or anticipating bearing failure.<br />Oil analysis tests<br />Symptoms Possible cause Test Cost<br />Viscosity Water or high Water content Low<br />temperature ASTM 445 viscosity Low<br />ASTM 974 neutralization number Low<br />ASTM 664 neutralization number Moderate<br />Viscosity change<br />colour change<br />Oxidation<br />ASTM 2296 neutralization number Moderate<br />Spectroscopy Low<br />Particle count Moderate<br />Direct reading ferrography Moderate<br />Particles Bearing<br />deterioration or<br />foreign matter<br />Analytical ferrography High<br />Question and answers Electrical Maintenance Unit<br />- 196 -<br />Question and answers Electrical Maintenance Unit<br />- 197 -<br />Important test on electrical equipments<br />1. Tests on transformer.<br />Test Purpose Item Required condition of machine<br />IR value<br />And<br />PI value.<br />Detects serious flaws,<br />moisture absorption and<br />cleanliness of winding.<br />Winding. Winding has to be isolated.<br />Tan delta or<br />dielectric<br />loss or<br />power factor<br />or HV test.<br />Indicates insulation<br />deterioration,<br />contamination and<br />physical damage.<br />Winding,<br />oil and<br />bushings.<br />Winding has to be isolated, oil<br />sample should be collected.<br />Excitation<br />current at<br />high voltage.<br />Indicates defects in the<br />magnetic core structure,<br />shifting or windings,<br />failures in turn to turn<br />insulation.<br />Winding Winding has to be isolated.<br />Turns ratio Indicates short circuited<br />turns and internal<br />connections<br />Winding Winding has to be isolated<br />Winding<br />resistance<br />Detects poor<br />connections and<br />conductor shorts<br />Winding Winding has to be isolated<br />Core IR and<br />inadvertent<br />grounds<br />Indicates deterioration<br />of core insulation<br />system<br />Core Winding has to be isolated<br />Water<br />contents<br />Indicates moisture level<br />in oil<br />Oil Oil sample has to be collected<br />Total acidity,<br />neutralization<br />number<br />Measures organic and<br />inorganic acids<br />Oil Oil sample has to be collected<br />Dissolved<br />gas analysis<br />Indicates specific gases<br />generated<br />Oil and<br />winding<br />Oil sample has to be collected<br />Furanite<br />compounds<br />Indicates cellulose<br />degradation<br />Winding Oil sample has to be collected<br />Question and answers Electrical Maintenance Unit<br />- 198 -<br />Question and answers Electrical Maintenance Unit<br />- 199 -<br />2. Test on Circuit breakers<br />Test Purpose Item Required condition of machine<br />IR value Detects serious flaws,<br />moisture absorption and<br />cleanliness.<br />Overall<br />insulation<br />system<br />CB has to be isolated<br />Dielectric<br />loss or tan δ<br />Indicates insulation<br />deterioration,<br />contamination and<br />physical damage<br />Overall<br />insulation<br />system<br />CB has to be isolated<br />DC HV test<br />(optional)<br />Determines condition of<br />insulation<br />Overall<br />insulation<br />system<br />CB has to be isolated<br />Contact<br />resistance<br />measurement<br />Detects poor contacts Contacts CB has to be isolated<br />Timings Detects faulty dashpots,<br />faulty adjustments,<br />weak accelerating<br />springs, defective shock<br />absorbers, buffers and<br />closing mechanisms, or<br />broken parts<br />Overall<br />breaker<br />CB has to be isolated<br />3. Tests on power cables<br />Test Purpose Component Required condition of machine<br />IR value Detects serious flaws,<br />moisture absorption<br />and cleanliness<br />Overall<br />insulation<br />system<br />Cable has to be isolated<br />Dielectric<br />loss or tan δ<br />Shows insulation<br />deterioration,<br />contamination and<br />physical damage<br />Overall<br />insulation<br />system<br />Cable has to be isolated<br />DC step<br />voltage test<br />Determines condition<br />of insulation<br />Overall<br />insulation<br />system<br />Cable has to be isolated<br />Resistance<br />of bolted<br />connection<br />Detects poor<br />connections<br />Bolted<br />connection<br />Cable has to be isolated<br />Question and answers Electrical Maintenance Unit<br />- 200 -<br />Question and answers Electrical Maintenance Unit<br />- 201 -<br />4. Tests on surge arrestor<br />Test Purpose Component Required condition of machine<br />IR value Detects serious flaws,<br />moisture absorption<br />and cleanliness<br />Overall<br />insulation<br />system<br />Arrestor has to be isolated<br />Watts loss<br />test<br />Shows insulation<br />deterioration,<br />contamination and<br />physical damage<br />Overall<br />condition<br />Arrestor has to be isolated<br />Resistance<br />of bolted<br />connection<br />Detects poor<br />connections<br />Bolted<br />connection<br />Arrestor has to be isolated<br />5. Tests on HV motors<br />Test Purpose Item Required condition of machine<br />IR and PI<br />value<br />Detects serious flaws,<br />moisture absorption and<br />cleanliness of winding<br />Stator and<br />field<br />Winding has to be isolated<br />Tan delta<br />or power factor<br />test<br />Evaluation of stress<br />grading, dielectric losses<br />and homogeneity of the<br />winding insulation<br />Stator<br />winding<br />Winding has to be isolated<br />DC Winding<br />resistance<br />Detects poor connections<br />and conductor shorts<br />Stator and<br />field winding<br />Winding has to be isolated<br />AC Impedance<br />on poles test<br />Detects the presence of<br />short circuit turns<br />Field<br />winding<br />Winding has to be isolated<br />DC HV step<br />voltage or<br />leakage current<br />test<br />Detects insulation<br />weakness and possibility<br />or warning of breakdown<br />of incipient fault<br />Stator<br />winding<br />Winding has to be isolated<br />Surge voltage Determines healthiness of<br />turn insulation<br />Stator<br />winding<br />Winding has to be isolate<br />Partial<br />discharge or<br />corona or TVA<br />probe.<br />Evaluation of stress<br />grading system and<br />location of partial<br />discharge sites<br />Stator<br />winding<br />Winding has to be isolated and rotor<br />has to be threaded out<br />ELCID test<br />(optional)<br />Determines healthiness of<br />stator core inter<br />lamination insulation<br />Stator core Rotor has to be threaded out<br />Online motor<br />current<br />signature<br />analysis<br />To determine the<br />healthiness of the motor<br />by giving all the electrical<br />parameters, harmonic<br />analysis, rotor bar heath<br />and bearing problems<br />Motor Online condition<br />Wear debris Bearing condition Bearing Bearing oil or grease sample has to<br />Question and answers Electrical Maintenance Unit<br />- 202 -<br />analysis for oil<br />or grease<br />assessment be collected<br />Question and answers Electrical Maintenance Unit<br />- 203 -<br />6. Tests on HV generator<br />Test Purpose Item Required condition of machine<br />IR and PI Detects serious flaws,<br />moisture absorption and<br />cleanliness of winding<br />Stator and<br />field<br />winding<br />Bus bar and neutral connection<br />has to be isolated<br />Tan delta or<br />power factor<br />test<br />Evaluation of stress<br />grading, dielectric<br />losses and homogeneity<br />of the winding<br />insulation<br />Stator<br />winding<br />Bus bar and neutral connection<br />has to be isolated<br />DC winding<br />resistance<br />Detects poor<br />connections and<br />conductor shorts<br />Stator and<br />field<br />winding<br />Bus bar and neutral connection<br />has to be isolated<br />DC step<br />voltage or<br />leakage<br />current test<br />Detects insulation<br />weakness and<br />possibility of warning of<br />breakdown of incipient<br />fault<br />Stator<br />winding<br />Bus bar and neutral connection<br />has to be isolated<br />Partial<br />discharge or<br />corona or<br />TVA probe<br />Evaluation of stress<br />grading system and<br />location of Partial<br />Discharge sites<br />Stator<br />winding<br />Bus bar and neutral connection<br />has to be isolated. Stator slot<br />exits are be accessible and if<br />necessary rotor has to be<br />threaded out<br />ELCID test Determines healthiness<br />of stator core inter<br />laminar insulation<br />Stator core<br />insulation<br />Rotor has to be threaded out in<br />TG’s, where as rotor poles has to<br />be removed minimum in case of<br />HG’s.<br />Wedge<br />tightness<br />check<br />Determines wedge<br />tightness<br />Stator<br />wedge<br />Rotor has to be threaded out<br />AC<br />impedance<br />test<br />Detects the presence of<br />short circuit turns<br />Field<br />winding<br />Rotor winding should be isolated<br />from the excitation system<br />Recurrent<br />surge<br />oscillograph<br />Detects interturn and<br />earth faults in winding<br />Field<br />winding<br />Rotor winding should be isolated<br />from excitation system. Test can<br />be carried out without threading<br />out the rotor also<br />O.C.C Detects shorted turns Field<br />winding<br />Online test<br />Thermal<br />sensitivity<br />test<br />Detects vibration cause Rotor Online test<br />Partial<br />discharge<br />To assess de-lamination,<br />stress control and slot<br />Stator<br />winding<br />PDA coupling coils has to be<br />fixed to the machine<br />Question and answers Electrical Maintenance Unit<br />- 204 -<br />analysis support tightness<br />Question and answers Electrical Maintenance Unit<br />- 205 -<br />ELECTRICAL SYSTEM<br />• What are the design objectives of Electrical System?<br />a. To evacuate generated electrical power.<br />b. To provide required power to SUT, UT, DG, UPS, and CUPS.<br />c. To provide required emergency power from onsite DG, UPS &amp; CUPS.<br />d. To provide Fast transfer in event of Class IV failure. Emergency transfer in<br />events of Class III and Class II failure.<br />e. Load shedding in event of one DG available.<br />f. To provide un-interruptible or few milli seconds interrupted power supply by<br />UPS and un-interruptible power supply by CUPS.<br />g. To provide operational flexibility by providing required qualities of requirement.<br />h. To provide isolation, Alarms, indication, protection of the system.<br />i. To provide fire protection.<br />j. To provide surge and lightning protection.<br />k. To provide adequate lighting.<br />l. To provide equipment earthing, system earthing, and personnel protection.<br />m. To provide necessary electrical and physical isolation of electrical equipments.<br />• What are the design guidelines for electrical system?<br />a. All safety related equipments are in control building, SRPH and are designed for<br />SSE conditions. As per studies seismic condition is not there within 5 kms and<br />nearest zone is away from 20 kms.<br />b. Safety related equipments are separated from suitable fire barriers of 3 hrs rating<br />by horizontal and vertical clearances and from turbine building which are houses<br />high energy rotating equipments and where potential for fire is exist.<br />c. Separate switchyard control is provided in case of non-availability of main<br />control room with line and bus coupler protection and bus bar protections.<br />Control room posses SUT, UT, GT, Generator and all classes of power supply<br />control and protections.<br />d. Protection panels of Generator, GT, and UT are separated from SUT in physical<br />to have system flexibility.<br />e. SCADA is provided in CER, TB and in switchyard separately.<br />f. EMTR for each A and B groups are separated.<br />g. Control supply for switchyard is separated from operating island.<br />h. To reduce fault level in lighting circuits separate 280-kVA transformer is<br />provided.<br />Question and answers Electrical Maintenance Unit<br />- 206 -<br />MAIN GENERATOR AND IPBD<br />• How output of the generator is depends?<br />Output of the generator is the function of volume, length, dia, airgap, and speed.<br />• What you mean by Gas pickup method?<br />Sucking cooled hydrogen gas from the air gap, circulating in canals and removing<br />the heat from the rotor. While sucking the gas, gas comes through the stator parts<br />also and removes the heat from the stator parts. The heated gas circulated to the<br />hydrogen for cooling purpose by the fans installed at rotor shaft. Thus the stator and<br />rotor of the generator is cooled. This method is called Gas pickup method.<br />• When the hydrogen explosion will take place in main generator?<br />When hydrogen concentration in air is more than 4% and less than 74% causes the<br />explosion.<br />• Can we excite generator without hydrogen? No<br />• What are the advantages of stator water?<br />a. High thermal capacity<br />b. Low electrical conductivity (Good insulator)<br />c. Low viscosity<br />d. Free of fire risk and non-toxic<br />e. Simple heat exchanger i.e. it can be circulate easily and cooled by heat exchanger<br />• How rotor windings are held in position against centrifugal force?<br />Rotor windings are held by duraluminium wedges and by non-magnetic steel<br />retaining rings in the overhang portion.<br />• What is the purpose of current carrying bolts in rotor?<br />Feeding DC current from slip ring to rotor winding.<br />• How rotor cooled?<br />Hydrogen picked up from stator core backspace, passes through ventilation canals on<br />rotor and comes out through adjacent canals. Shaft fans aid the hydrogen flow. Heat<br />from the hydrogen removed by 4 nos. of hydrogen coolers. (NAPW)<br />• What is rotor E/F relay setting?<br />1.0 mA<br />• Are we using DCCB in the plant?<br />Question and answers Electrical Maintenance Unit<br />- 207 -<br />Yes, generator field breaker<br />• How arc is quenched in Generator Field Breaker?<br />By magnetic blow out coils, arc is elongated very fastly, so resistance of arc<br />increases, soon becoming unstable and quenched by arc chutes.<br />Question and answers Electrical Maintenance Unit<br />- 208 -<br />• How generator is protected from switching surges and lightning surges?<br />Surge capacitor and lightning arrestor.<br />• What is the use of generator PT’s?<br />AVR, Protection &amp; Metering.<br />• What is the difference between PT and normal transformer?<br />Burden of PT is less and burden of normal transformer is high.<br />• What is the % overload allowed for TG and DG?<br />For TG nil<br />For DG 110% for 2 hours.<br />• When TG works as induction generator?<br />When excitation alone lost.<br />• Why motoring should be prevented in TG and DG?<br />In TG motoring prevented due to the turbine limitation otherwise last stage blade<br />will fails.<br />In DG motoring prevented, because of unburned fuel catches fire in DG.<br />• Why GFB closed only after reaching rated speed?<br />To prevent over fluxing of transformers or generator.<br />Emf = 4.44 f φ Z A<br />If f frequency is reduced due to less speed,<br />φ = V / f Z A<br />And flux will be more to saturate the core of transformers or generator.<br />• What are the protective parameters to changeover AVR auto to manual?<br />a. PT supply fail.<br />b. Auto pulse fail.<br />c. Supply of limiter fail.<br />d. Supply of auto channel fail.<br />e. Regulated supply fail.<br />f. High auto reference.<br />• Why AVR changes over to manual on AVR PT fuse failure?<br />Because loss of feedback to voltage corrector.<br />• Will main generator differential relay pick up for generator earth faults?<br />No earth fault current limited to 5A, while differential setting is 10%.<br />Question and answers Electrical Maintenance Unit<br />- 209 -<br />• Why generator differential provided?<br />For generator phase to phase faults and 3 phase faults.<br />Question and answers Electrical Maintenance Unit<br />- 210 -<br />• What is the effect of loss of excitation on generator?<br />Large induced currents in rotor leads to rotor end part over heating.<br />Leading VAR taken from grid leads to severe voltage dips in grid, if grid is weak.<br />Stability of machine lost.<br />Stator overheating.<br />Machine speed rises slightly.<br />• What are the limiters provided in excitation system?<br />a. Rotor current limiter (3000 A)<br />b. Rotor angle limiter (75° lag)<br />c. Under excitation limiter.<br />d. Stator current limiter (lead 10000 A)<br />e. Stator current limiter (lag 10000 A)<br />f. N – 2 limiter.<br />• What is the effect of unbalance currents in generator?<br />Double frequency currents are induced in metal parts of rotor and overheating of<br />retaining rings and non-magnetic wedges.<br />• Why low forward power relay used in parallel to reverse power relay?<br />A small steam leak through CIES valves will keep the machine floating on to grid at<br />low power. So even if turbine trip, relay picks up, there is no trip actually. So low<br />forward power relay (0.54%) used to detect the condition.<br />• Why earth fault current of generator should not be reduced below 5A?<br />To limit over voltage due to neutral shift.<br />• Why not high resistance for earth fault than using grounding transformer &amp; resistor<br />0.45 ohms?<br />It is mechanically unwide. Difficult to manufacture.<br />• Why starting resistor is provided in barring gear motor?<br />Starting resistance at stator reduces voltage at stator terminals and accelerates the<br />motor very slowly so as to allow smooth gear engagement.<br />Other methods are<br />a) Auto transformer.<br />b) Star-delta starter.<br />• Will rotor earth fault relay operate for earth fault in main exciter winding or RCU<br />Diode Bridge also?<br />Yes.<br />Question and answers Electrical Maintenance Unit<br />- 211 -<br />• Why neoprene rubber bellows in generator IPBD?<br />Prevent vibrations transmitted from generator to IPBD.<br />• How moisture entry is prevented into bus duct?<br />Silicagel breathers at either end.<br />Question and answers Electrical Maintenance Unit<br />- 212 -<br />• Why aluminium bus duct is used?<br />Aluminium is nonmagnetic material.<br />Short circuit forces are less.<br />• Where fault level is more, whether in generator bus duct or UT bus duct? Why?<br />Fault level is more in UT bus duct. Because the fault currents fed by the both<br />generator and GT adds up within UT bus duct in case there is a fault in UT bus duct.<br />• Which is better, whether<br />a) Phase segregated bus duct or<br />b) Common bus duct?<br />Phase segregated bus duct is better, since phase to phase faults are avoided.<br />• Why cannot we have cables instead of bus duct in main generator?<br />Very large number of cables in parallel required problems of sealing the<br />terminations.<br />• What is the material of slip ring?<br />Alloy steel<br />• Why rotor impedance testing done during static and running condition?<br />To detect rotor earth fault.<br />• Which parameter indicates the rotor short-circuited turns (Not involving earthfault)?<br />Vibration increases.<br />• How stator water purity is held?<br />Filters, Vacuum pumps, Expansion tank, and magnetic filter.<br />• What are the routines checks on slip rings?<br />a. Correct mV drops brush to be used.<br />b. Brush tension adjustment.<br />c. Air cleaning to reduce leakage current.<br />d. Brush bedding before use.<br />e. Field polarity change every 6 months.<br />• Can we trip GFB from control room during unit operation?<br />No only if generator breaker is off.<br />• What is the voltage and frequency limit of generator?<br />±5%, ±5%<br />Question and answers Electrical Maintenance Unit<br />- 213 -<br />• What is the negative sequence capability?<br />I2 = 5% max,<br />I2<br />2 t = 7<br />Question and answers Electrical Maintenance Unit<br />- 214 -<br />• Why should we keep the brushes released during long shutdown?<br />Brushes wear out unevenly, when run on barring gear speed.<br />• How hydrogen purity reduces?<br />Due to seal oil vapour mixing.<br />• What is the purpose of back up impedance protection in main generator?<br />Covers inter-phase faults both externals to the GT and in GT. This also covers<br />partially faults inside generator, time delayed to coordinate with 230 kV-distance<br />protection.<br />• Why alternator rotor is made of solid iron?<br />Because, rotor flux = DC continuous<br />No iron loss problems.<br />• Why 50 Hz chosen?<br />Earlier 25 Hz generally used.<br />After developing of the high-speed turbine, 50/60 Hz standardized.<br />• Why oil cannot be used instead of water in generator stator?<br />Oil has high thermal capacity than gas, but low thermal capacity than water. Ability<br />to absorb heat is also less than waters.<br />High viscosity of oil causes linear flow and poor surface heat transfer in small ducts.<br />Large pumping power required.<br />• What is the purpose of JOP?<br />Lifts the rotor by injection of oil at high pressure, when BGM is in service. Outlet<br />pressure of JOP is 140 kg/cm2.<br />• What is the purpose of lubrication oil?<br />Keep oil film in bearings, avoid metal to metal contact between bottom of journal<br />and bearings avoid damage to bearings by lubricating the bearings. This also<br />removes heat from that part.<br />• What are the purposes of barring gear?<br />Start rotor from rest.<br />Eliminates sag in rotor - straighten and avoid rubbing at glands.<br />Avoid direct contact journals and bearings.<br />Avoid differential temp between top and bottom of cylinder due to convection of<br />Steam or hot air inside turbine cylinders.<br />Question and answers Electrical Maintenance Unit<br />- 215 -<br />• How shaft voltage produced by turbine?<br />Due to un-symmetry in the flux path of core, non-uniform air gap, un-symmetry in<br />the rotor magnetic field during short circuit in the rotor winding, causes voltage to<br />develop across the ends of rotor shaft.<br />Question and answers Electrical Maintenance Unit<br />- 216 -<br />• Why shaft-earthing brush is different from normal brush?<br />The contact resistance should be very low for shaft earthing brush, to prevent even<br />small current through the oil film, so used copper magnite brush or silver carbon<br />brush.<br />• What is the necessity of shaft voltage measurement?<br />It is to observe insulation of bearing 6 &amp; 7 and hydrogen seal assembly. It requires<br />minimum leakage current (<100mA) through bearings and shaft seals to avoid pitting<br />of bearings.<br />If the leakage current >100mA, clean the insulation provided between bearing<br />pedestal and seal housing with earth.<br />• Why generator stator having alternate arrangements of hollow and solid conductor?<br />It ensures an optimum solution for increasing current and to reduce losses.<br />• What type of insulation is done for stator bars?<br />Bar insulation is done with epoxy mica thermosetting insulation. This insulation is<br />void free and possesses better mechanical properties. This insulation is more reliable<br />for higher voltages. Conductors are provided with glass lapped strand insulation.<br />After curing the insulation the epoxy resin (glue) fill all voids in the insulation.<br />• How carona discharge is prevented in generator insulation?<br />To prevent carona discharges between insulation and the wall of the slot, the<br />insulation in slot portion is coated with semi conducting varnish. This eliminates the<br />formation of creepage sparks during operation and during HV test.<br />• Why Generator should run within capability region?<br />Operating the Generator in excess of the capability curves will causes increase in<br />copper temperature, thermal expansion and higher insulation stresses.<br />• How cooling is done for slip ring and brush gear?<br />A centrifuge fan is mounted on the shaft in between two slip rings for ventilation of<br />the slip rings and brush gear.<br />• What is the type of brush used in brush gear?<br />Low co-efficient of friction and self-lubricating morganite grade carbon HM100.<br />Now a day we are using LFC554 for economical reasons.<br />• What is the name of instrument used to measure conductivity?<br />Gas chromato graph.<br />Question and answers Electrical Maintenance Unit<br />- 217 -<br />• What is the need of staggering of brushes and helical grooves?<br />The need of staggering is for uniform wear of brush and slip rings. The helical<br />grooved are provided to improve the brush performance by breaking air pockets. The<br />forced ventilation fan removes carbon dust from the helical grooves.<br />Question and answers Electrical Maintenance Unit<br />- 218 -<br />• What is the purpose of shaft earthing and bearing insulation?<br />The voltage generated in the shaft due to the leakage fluxes can circulate current<br />through the shaft. If shaft earthing is not done the leakage current will flow through<br />the bearings to ground and pitting of bearings will result. Hence bearing foundation<br />and pipelines are insulated.<br />• What is the purpose of POLARIZATION INDEX (PI) value?<br />It is used to assess the degree of dryness of windings. It depends on free ions in<br />insulating material. Initially for a new insulator free ions are less and hence more<br />resistance will be more. For old insulation initially free ions will be more depends on<br />age and material and hence resistance will be less. So the PI value for new insulation<br />will be more and for old insulation it will be less.<br />• What is the requirement of stator water electrical conductivity?<br />The cooling water must have an electrical conductivity less than 2.5 micro mho/cm.<br />One portable polishing unit consisting of mixed bed is also provided in the system to<br />remove impurities and maintain stator water conductivity at a less value.<br />• What is the necessity of Seal oil system?<br />The annular gap between stator and rotor of the generator are to be sealed to prevent<br />hydrogen leak from the casing.<br />Type of seal – ring type shaft seals<br />Pressure of seal oil – 4 kg/cm2<br />• What is the pressure of rotor gas (hydrogen)?<br />3.5 kg/cm2<br />• What is the paint used in the surface and interior of enclosure and why it is?<br />Matt black paint, for efficient heat dissipation.<br />• What is type of nut and bolts are used in IPBD?<br />Non-magnetic stainless steel nut and bolts are used in IPBD to restrict magnetic<br />effect at joints.<br />• Why flexible expansion joints are used in IPBD?<br />To cater thermal expansion and contraction due to heating and to eliminate<br />mechanical vibrations to the equipment.<br />Question and answers Electrical Maintenance Unit<br />- 219 -<br />• What are the salient features of IPBD?<br />a. This for a vital link between generator, GT, UAT, SPPT, SET and neutral<br />grounding transformer.<br />b. The continuous enclosure operating at ground potential limits the leakage flux<br />outside the enclosure to a very low value thereby eliminating the problem of<br />inductive heating of magnetic materials in the vicinity of the busduct.<br />c. Shielding effect of the enclosure reduced the electromagnetic forces under fault<br />conditions between bus to bus to a great.<br />d. The IPBD consists of high purity aluminium alloy bus supported by high strength<br />porcelain insulator (24 kV class) within enclosure separates adjacent conductor<br />by air. This eliminates phase to phase faults to a great extent.<br />e. Practically negligible inductive heating on adjacent steel structure.<br />f. High current carrying capacity. Because the conductors are of circular type<br />having very little skin effect and has a very large cooling surface.<br />g. Conductors are painted with epoxy Matt black paint results in heat dissipation and<br />the temperature rise is small and current carrying capacity is improved.<br />h. High dielectric strength as conductors are supported on porcelain insulators.<br />i. Air tight, watertight and dust free bus conductors. Hence maintenance is nil.<br />j. Separate parts erected IPBD. Hence changing parts makes it easy.<br />• Why neoprene rubber bellows are used in IPBD?<br />Neoprene rubber bellows are used near the terminals of the equipment and also at<br />building wall from indoor to outdoor area to allow thermal expansion and to<br />minimise vibrations.<br />• Why aluminium bus bars are silver-plated in IPBD?<br />Aluminium bus bars are silver-plated at flexible connection to prevent the galvanic<br />corrosion ant also for low contact resistance.<br />• Why seal-off bushings are used in IPBD?<br />To prevent interchange of air at different temperature and leakage of hydrogen or<br />infiltration of dust into the bus duct.<br />• What are the precautions to be taken while working at SPPT?<br />PT trolley should be isolated very carefully so as to isolate secondary terminals first<br />and primary (HT) terminals next. When primary isolated the arrangement in the<br />trolley make ground connection and HT terminal will be discharged at the drawn-out<br />position.<br />When fuse is blown the temporary earth should be done at the HT side of the fuse to<br />replace the fuse. Because PT may be energised through secondary side.<br />Question and answers Electrical Maintenance Unit<br />- 220 -<br />• What is use of hot air blower in IPBD?<br />To remove moisture and to prevent moisture condensation inside the duct at<br />commissioning time or in long shutdown periods.<br />Question and answers Electrical Maintenance Unit<br />- 221 -<br />• What are the temperature limits for UAT and SPPT bus bar?<br />2 kA (UAT) &amp; 1 kA<br />Ambient temp 45°C 45°C<br />Maximum temp 60°C 60°C<br />Short circuit for 1 second temp 200°C (max load) 200°C(max load)<br />Bus material Al alloy Al alloy<br />Thickness 15 mm 6 mm<br />Dia 12.7 cm<br />• Specification of NGT &amp; NGR.<br />NGT – 1 phase, natural cooled, indoor dry type, 16.5 kV / 250V, 50 kVA.<br />NGR – natural cooled, stainless steel grid type, 0.5Ω, 250V, 288A (continuous) and<br />temperature rise allowed to 375 °C.<br />• Surge protector and potential transformer cubicle specification.<br />Surge protector – non-inflammable, synthetic liquid impregnated and hermetically<br />sealed, 24KV, 0.25μ f (micro farad).<br />PT – 16500/√3 /110/√3 volts. Fuse – 24kv, 3.15A.<br />• How the power of the Generator can be varied?<br />Injecting inlet steam to the prime mover can vary active power. Reactive power can<br />vary by the Generator main field voltage variation. An excitation change PF at which<br />load is delivered.<br />Active power is produced by source and used effectively. VAR is the power used for<br />magnetization of core of transformers, motors, generators, overhead transmission<br />lines (capacitive), household appliances etc.<br />• What is the protection for IPBD?<br />Generator – GT overall differential protection.<br />• How the liquid in generator can be detected?<br />There are three liquid detection devices provided for the same purpose.<br />• Why and where the magnetic filter is provided in stator water circuit?<br />Magnetic filter is provided to catch the metal particles in stator water circuit, which<br />are produced in the pipelines. This is mounted at the end of the circuit nearer to the<br />inlet of the generator.<br />• What are the isolations required for working on IPBD/ Generator?<br />a. Generator field breaker open and tagged.<br />b. GT breaker open and earth switch closed.<br />Question and answers Electrical Maintenance Unit<br />- 222 -<br />c. Barring gear motor stopped and tagged.<br />d. Generator PT’s isolated and tagged.<br />e. CB 472 and CB 474 open and PT’s are isolated and tagged.<br />f. Before doing any work on brush gear 64F1 relay to be taken out.<br />Question and answers Electrical Maintenance Unit<br />- 223 -<br />• What are the futures of turbine generator?<br />a. Low heat drop<br />b. Moisture control (HP-0.26%, LP-3%.)<br />c. Turbine governing system<br />d. 70% steam dumping to the condenser to avoid reactor trip.<br />e. Gland sealing<br />f. LP exhaust hood cooling<br />g. Generator stator and rotor cooling<br />h. Hydrogen sealing<br />i. Static excitation<br />• What are the intervals for generator overhauling?<br />a. 1st inspection after 8000 hrs of working<br />b. 2nd inspection after 8000 hrs of 1st inspection<br />c. 3rd inspection after 24000 hrs of 1st inspection<br />d. 4th inspection after 48000 hrs of 1st inspection<br />• Write nameplate details of the main generator.<br />Type THW-235<br />kW 237700<br />kVA 264000<br />Voltage 16500 V<br />Amps 9240 A<br />Power factor 0.9 lag.<br />Field voltage 326 V<br />Field current 2755 A<br />Insulation Class-F<br />Speed 3000 rpm<br />50 Hz, 3, double star connection.<br />• What are the torque settings used in IPBD connection?<br />M12 (Nut bolt) 4506 100 kg-cm or 55 NM<br />M16 (Nut bolt) 9006 250 kg-cm or 80 NM<br />M20 (Nut bolt) 18006300 kg-cm or 100 NM<br />• Write critical speeds of turbine generator?<br />Generator rotor<br />1st critical speed 1283 rpm<br />2nd critical speed 3600 rpm<br />Combined turbine generator<br />1st critical speed 1938 rpm<br />2nd critical speed 2120 rpm<br />Question and answers Electrical Maintenance Unit<br />- 224 -<br />3rd critical speed 2385 rpm<br />4th critical speed 2837 rpm<br />Question and answers Electrical Maintenance Unit<br />- 225 -<br />• What type of governing system used in turbine and what are the purposes of the<br />system?<br />Hydraulic governing system of centrifugal (speed) governer type is adopted in<br />turbine.<br />Sensitive oil pressure to actuate centrifugal governer is 6.1 kg/cm2 (max). At 6.7<br />kg/cm2 relief valve is attached for on load testing.<br />Relay oil at pressure 21 kg/cm2 (max) is used to actuate HP CIES valve, governer<br />valve, LP CIES valve and LP governer valves.<br />At speed of 2560-rpm governer system becomes effective and starts draining of<br />sensitive oil to 2.81 kg/cm2 as speed is 2760 rpm and this is the governer take over<br />speed. Once the speed takes over by governer, governer valves position comes to<br />closing side and then CIES valves are opening fully. At this stage further opening of<br />CIES valve does not change any speed of system and the speed depends only on<br />governer valve opening position and speeder gear system.<br />HP speeder gear controls HP governer valves and LP speeder gear controls LP<br />governer valves and closes fully when 6% over speed which starts when 3% over<br />speeding.<br />The main purposes are as follows.<br />a. Bring the TG to rated (synchronous speed) speed from rest.<br />b. Loading and unloading when synchronised.<br />c. Responding with grid frequency variations within design rage and loading and<br />unloading the machine so that grid frequency remains stable.<br />d. Limiting the load as per reactor load.<br />e. Protecting the machine from over speed and from sudden large load thrown off or<br />trip.<br />f. Tripping the machine and bringing it on barring gear when event for not operation<br />happens.<br />g. When synchronised the speed is regulated by speeder gear from BPC signal. Once<br />synchronised the grid frequency and speeder gear controls the speed.<br />• Why inter-turn protection is provided along with differential protection in generator?<br />Inter-turn protects two separate windings from the fault of the generator.<br />• How patina formation is done.<br />By injecting low excitation current of 50 Amps for half an hour interval to 250<br />Amps. (Epoxy insulation in the stator winding absorbs no moisture).<br />Question and answers Electrical Maintenance Unit<br />- 226 -<br />• What are the tests to be carried during PM checks of IPBD?<br />a) Physical inspection of bus for any spark or overheating or discoloration.<br />b) Physical inspection of copper braided flexibles for discoloration.<br />c) Physical inspection of inspection window gaskets, seal off bushings, supporting<br />insulators, CT’s, painting of IPBD.<br />d) Torque tightness of flexibles.<br />e) Connection tightness of CT’s, SPPT cubicle, NGT cubicle, CT’s master JB, and<br />Generator terminal bushing connection.<br />f) Tightness of supporting insulator, seal off bushing, inspection windows<br />g) Inspection of rubber bellows<br />h) Electrical checks on SPPT, NGT, CT, mVDT of copper flexible connection.<br />i) Capacitance measurement of surge capacitor.<br />j) Healthiness checks of lightning arrestor.<br />k) HV test of IPBD<br />l) Tan-delta test of IPBD<br />m) IR value measurement<br />n) Cleanliness checks entire IPBD.<br />• What are the works to be done in generator in major overhauling?<br />Works on stator<br />a) Hydro test (DM water at 5 kg/cm2 pressure, leak acceptable is 5% for 24 hrs).<br />b) Hydro test of H2 coolers (DM water at 4 kg/cm2 for 30 seconds no leak is<br />allowed)<br />c) Pneumatic test with mask air.<br />d) Drying out of stator conductor. Hot air blower is used.<br />e) IR value check.<br />f) Stator overhang portion inspection.<br />g) Inspection of Teflon tubes and rubber grummets.<br />h) Stator wedge tightness test with 200 grams hammer.<br />i) Inspection of RTD’s.<br />j) Maintenance of end shields.<br />k) Hot air and hot water test of stator conductors to check whether flow through all<br />stator conductors is uniform.<br />l) Measurement of IR and PI value.<br />m) Capacitance and tan-delta measurements.<br />n) Partial discharge test.<br />o) Winding resistance measurement.<br />p) DC step voltage.<br />q) ELCID (electromagnetic core imperfection detection) test.<br />Question and answers Electrical Maintenance Unit<br />- 227 -<br />Works on Rotor<br />a) Nitrogen leak tightness test of CC bolts at 4 kg/cm2.<br />b) Inspection of rotor slots.<br />c) Purge test of rotor ventillation canals.<br />d) DP test on slip-ring hub to detect micro crack.<br />e) DP and Ultrasonic test on retaining rings to detect any cracks.<br />f) Measurement of IR and PI value.<br />g) Impedance measurement.<br />h) Recurrence surge oscillograph.<br />i) Winding resistance measurement.<br />j) Slip-ring groove cutting and machining.<br />k) Patina formation. Then OCC test.<br />• What is the purpose of tan-delta measurement?<br />Insulation in electrical system has parameters such as Capacitance, Die-electric loss,<br />and Power factor. By detecting the changes in these parameters failures can be<br />revealed. In this tan-delta test measured quantities are dissipation factor, power<br />factor, capacitance and dielectric power loss.<br />The very purpose of this test is to detect moisture content in the insulation. This<br />detects moisture and void in the insulation. This indicates amount of ionization.<br />• What is meant by partial discharge? How can be tested?<br />Partial discharges are electrical sparks, which occur in gas voids within the insulation<br />when the voltage is high enough. The discharges are partial since there is some<br />insulation remaining to prevent a complete breakdown. Partial discharge can erode<br />the insulation and therefore contribute to insulation ageing.<br />This can be tested by electromagnetic probe, which is a detector that is sensitive to<br />the radio frequency signals produced by the partial discharges within the winding.<br />With this probe test it is possible to locate specific sites of deterioration within the<br />winding when the winding is energized.<br />Question and answers Electrical Maintenance Unit<br />- 228 -<br />• What is the purpose of HV / leakage current test on stator?<br />This is to find out weakness in the stator winding. If the ambient conditions are right,<br />and the insulation is weak, the leakage current will increase non-linearly.<br />The record of voltage versus leakage current provides the condition of the winding<br />for present and future use and may permit prediction of breakdown voltage whether<br />it is within or slightly above the test voltage.<br />HV test voltage = 1.5 * rated voltage for AC<br />And DC voltage = (2E + 1 kV) 1.6<br />Where E – rated voltage.<br />1.6 – AC/DC conversion factor.<br />Application of HV voltage also depends on the age factor or condition of the<br />machine.<br />Following are the findings of HV test.<br />a) Capacitance charging current.<br />b) Dielectric absorption current.<br />c) Surface leakage current.<br />d) Partial discharge current.<br />e) Volumetric discharge current.<br />In HV test starting leakage current should be more than switching off current in<br />􀀜 Amps.<br />• What is Recurrent Surge Oscillograph (RSO) Test?<br />RSO test is performed to detect faults in rotor windings. The electrical faults in<br />generator rotors fall into two main categories. The faults from the winding to the<br />body and the faults between the parts of the winding (inter-turn faults). The existence<br />of the faults will frequently display excessive mechanical vibration and cause serious<br />concern.<br />• What is the purpose of rotor AC Impedance measurement?<br />Periodic measurement of rotor impedance using an AC power supply is another<br />means of detecting the presence of shorted turns in a rotor winding. Impedance<br />measurement is more sensitive than the resistance measurement for the detection of<br />shorted turns. This is because the induced backward current in a single shorted turn<br />opposes the MMF of the entire coil, thus greatly reducing the reactance.<br />Question and answers Electrical Maintenance Unit<br />- 229 -<br />• What is IR and PI value? Why it is measured?<br />IR It is the ratio of the DC voltage applied between the terminals and ground to the<br />resultant current. When the DC voltage applied three components flow,<br />a) A charging component flows into the capacitance of the winding.<br />b) A polarization or absorption current involving in the insulation molecular<br />mechanism.<br />c) A leakage component over the surface between exposed conductor and ground<br />which is highly dependent on the state of dryness of the winding.<br />The first two current components decay with time. The third component is<br />determined by the presence of moisture or ground fault and relatively constant with<br />time. Moisture may be with in the insulation or condensed on the end windings or<br />connection surfaces, which are often dirty. If this leakage current is larger than the<br />first two components then the total charging current will not change significantly<br />with time. Thus to determine how dry and clean the winding, IR is measured after<br />one minute and after 10 minutes. The ration of the 10 minutes reading over the oneminute<br />reading is called the PI (Polarization Index).<br />PI value detects relative condition of insulation with respect to moisture and other<br />contaminants.<br />• What is the generator IR value when generator is filled with stator water and<br />hydrogen?<br />Generator IR when filled with stator water and hydrogen is about 100 kΩ only. That<br />is because most of the gases and liquids are self-restoring insulators. As we are<br />measuring insulation with 1 kV or 5 kV megger, the ionic current or leakage current<br />will be same and the IR value will be approximately same. As we are increasing the<br />test voltage to higher value say to 100 kV the breakdown point will occur as in the<br />graph and insulators will breakdown or puncture.<br />Ionic current<br />Leakage<br />Current<br />Saturation region<br />Voltage (kV)<br />Water and hydrogen are self-restoring insulators. First we are measuring insulation<br />on 1 kV voltage i.e. 100 kΩ. As the field voltage and stator voltage raises the heat<br />produced in the stator and rotor will increase the IR value of the machine in running<br />condition.<br />So ionic current region needs 1 kV/cm, saturation region moderate voltage (1 kV to<br />70 kV), and breakdown region is above 70 kV voltage.<br />Question and answers Electrical Maintenance Unit<br />- 230 -<br />• What is the purpose of DC winding resistance test?<br />To detect the shorting of winding and loose or poor connection of the windings.<br />Question and answers Electrical Maintenance Unit<br />- 231 -<br />• What are the types of Grounding?<br />Equipment grounding is the grounding on non-current carrying metal parts. This is<br />done for personnel safety of the operator and for the equipment safety by blowing<br />the fuse when earth fault current flows through the fuse.<br />Neutral Grounding is done to protect the equipment against arcing grounds, to<br />protect system from lightning surges by passing surge current through the earth and<br />to protect against unbalanced voltage with grounds. When fault occurs the system<br />voltage increases ♦3 times. This gives stress on the system and failure of the<br />insulation if the neutral grounding not designed properly.<br />Mainly there are three types of neutral earthing<br />a) Directly<br />b) Resistance<br />c) Reactance<br />• What is Arcing Grounds?<br />When earth faults occurs, arc with the ground and phase will occur. The arc<br />extinguishes and restrikes as a repeated and regular manner. This is called Arcing<br />Ground.<br />• How neutral grounding adopted?<br />For above 3.3 kV and below 22 kV resistance grounding is preferred. In this voltage<br />level capacitive ground current is not large, so reactance grounding is not used.<br />For below 3.3 kV that for 415 V external resistance earthing is not necessary.<br />Because normal earthing (plate earthing) gives 1.5Ω resistance. This limits current to<br />E (R∅)<br />Ω<br />230/1.5=153A(Current limit without resistance).<br />For above 22 kV solid or direct grounding is used.<br />Reactance grounding is used where capacitive currents are large instead of resistance<br />grounding in transmission lines, generators etc. to neutralize capacitive current by<br />adding reactive current.<br />• How main generator earthing is done?<br />Generator neutral earthing is done through transformer and earth fault current is<br />limited through resistance, which is connected across the secondary of the<br />transformer. Generator 16.5 kV earth fault current is isolated from 220 kV through<br />GT. Only star point of the generator is grounded.<br />Question and answers Electrical Maintenance Unit<br />- 232 -<br />• How generator earth fault relay works?<br />100% earth fault relay works on the principle involving monitoring of neutral side<br />and line side components of 3rd harmonic voltages produced by generator in service.<br />Since the machine is grounded with reactance XL (transformer), a flow of 3rd<br />harmonic current is there in between ground and the machine neutral. Under healthy<br />condition the line and neutral impedance Z are fixed. Thus the 3rd harmonic voltage<br />(Vs) at machine line (VL3) and neutral end (VN3) should bear a constant ratio. When a<br />fault occurs in the machine winding the distribution of VL3 and VN3 undergoes a<br />change from that a healthy condition. In the extreme case if a fault occurring on the<br />machine neutral side, VN3 becomes zero and VL3 becomes Vs and vice versa.<br />The fault in Blind zone will be detected by VL3 neutral displacement module,<br />which is tuned to find frequency.<br />Blind zone<br />Neutral Line<br />Fault<br />Earth<br />• How generator protections are classified in nuclear power station?<br />Classification of generator protection in nuclear power station.<br />1. MAIN Protection 2. BACKUP Protection<br />Stator E/f Back up Impedance<br />Loss of Excitation Over Voltage<br />Pole slipping Under Freq.<br />Differential Over freq.<br />Inter turn 4. EXCITATION Protection<br />Unbalance current Excitation transformer over current<br />3. START UP Protection Rotor E/f. and Rotor o/v<br />Phase o/c during startup 48 V DC fail<br />E/F during startup more than 3 Bridge fail (¾ logic)<br />Manual channel fails<br />Transformer over temperature (Class –B)<br />• State torque formula.<br />Torque (T) = kT *S *IR *COSθ<br />Where T = Torque in pound – feet<br />kT = Torque constant.<br />S = stator flux<br />IR = Rotor current<br />cosθ = Rotor power factor<br />Question and answers Electrical Maintenance Unit<br />- 233 -<br />• How main generator protection grouped?<br />The various protections associated with the generator, Generator transformer and Unit<br />transformer are connected to a trip unit through trip relays 86A, 86B and 86C.<br />The protective levels of generator are in three classified groups Class A, Class B and<br />Class C protections which involve fault in the generator, Generator transformer and<br />requires high speed clearance are grouped under Class A. These are routed through<br />trip relays 86A. This trips generator transformer HV side breaker, generator field<br />breaker, and LV side breaker of UT and Turbine simultaneously.<br />Certain protections such as loss of excitation, negative sequence protection, overfluxing<br />etc., can tolerate sequential tripping of turbine followed by the generator such<br />that the entrapped steam in the turbine is fully spent before generator is tripped and<br />reduces the risk of over speeding of the turbine. These protections are classified as<br />Class B. These are connected to operate on trip relay 86BG. This relay initiates the<br />tripping of turbine (closure of stop valves) and also the LV side breaker of UT through<br />trip relay 86B1 and 86B2. After turbine stop valves are closed and the entrapped<br />steam is spent, the output power of the generator will come down and is sensed by<br />under power relay 32A and 32B. These interlocks are wired in series with the Class B<br />trip relay 86B, which is wired to trip the generator breaker, generator field. Obtaining<br />better security, the Under Power interlock circuits are duplicated. Some protections<br />such as Bus bar differential, generator under frequency etc, requires tripping only of<br />the 220 kV side of the generator transformer to isolate the external fault. These are<br />classified as Class C. These protections are wired to trip relay 86C, which initiate<br />only the tripping of the generator transformer HV side breaker. During Class C trip,<br />the generator will come on House load mode of operation.<br />Question and answers Electrical Maintenance Unit<br />- 234 -<br />• What are the generator protection parameters?<br />Generator protection parameters are as follows.<br />GENERATOR PROTECTION SCHEME<br />CLASS A1 CLASS A2<br />Gen. rotor earth fault (64F2) Stator O/C during start (50S ABC)<br />100% stator earth fault (64A) Stator E/F during start (64 C)<br />GT restricted earth fault (64 GT) Stator backup E/F (64 B)<br />UT restricted earth fault LV A (64 UT A) GT backup O/C (50/51 GT)<br />UT restricted earth fault LV B (64 UT B) GT backup E/F (51 N GT)<br />Gen. differential (87 G) Gen. backup impedance (21G – 1 ABC)<br />Gen. interturn (87 IT) Gen. field fail with U/V (27/40G)<br />GT overall differential (87 GT) UT backup O/C (51 UT ABC)<br />UT differential (87 UT A/B/C) UT backup E/F LV-A (51N LV-A)<br />Reverse power (37 G) UT backup E/F LV-B (51N LV-B)<br />GT buchholz, OLTC oil surge, fire (30 A/G/D) LBB protection (50 Z)<br />UT buchholz, fire (30 A/D)<br />Excitation O/C stage – 2<br />Rotor + &amp; - ve over voltage<br />Excitation 48 V DC fail<br />More than 3 bridge fails (3/4 logic)<br />CLASS B CLASS C<br />Gen. field failure without U/V (40 G) Gen. backup impedance stage – 2(21G – 2)<br />Gen. negative phase sequence (46 G/GT) Gen. pole slip (78G)<br />Gen. over frequency (81 – 3) Gen. under frequency (81 – 1 / 2)<br />GT over fluxing protection (99 GT) GT backup earth fault (51N GT)<br />GT oil temp / winding temp high (30 C/E)<br />UT oil temp / winding temp high (30 C/E)<br />Low forward power (32 B/A)<br />Turbine process parameter trip (86 BG)<br />Excitation transformer temp high<br />Manual channel fails<br />Excitation transformer O/C stage – 1<br />Regulation under test<br />Question and answers Electrical Maintenance Unit<br />- 235 -<br />• State class – B process side trip parameters.<br />Sl Parameter Normal Value Low Value High Value Trip Value<br />1. Reactor trip + 200 milli sec<br />2. Reheater steam Pr. High 5.4 kg/cm2 c 5.75 kg/cm2<br />3. Exhaust hood steams temp. 93°C 149°C<br />4. Lub. oil Pressure low [0.35 kg/cm2<br />5. Relay oil pressure low 21 kg/cm2 17.38 kg/cm2 [ 3.5 kg/cm2<br />6. Trust bearing <P high !9.114 kg/cm2<br />7. Condenser vacuum low 696.5 mm Hg 660 mm Hg 559 mm Hg<br />8. Stator water cond. High 5 μ Mho / cm 13.3μ Mho 20 μ Mho<br />9. Stator water flow low 30 M3 / hr 21 M3 / hr 17 M3 / hr<br />10. Boiler level high 2/3 trip<br />• What are the manual trips required from the generator side?<br />Quantity 1st ann. Action/2nd ann. Action<br />Bearing babbitt temp. high 75°C 80°C >80°C manual trip<br />Bearing outlet oil temp. high 60°C 65°C >65°C manual trip<br />Generator seal oil inlet temp 45°C >45°C manual trip<br />Presence of liquid in Gen. Manual trip<br />DM water outlet temp 85°C Unload >85°C Rundown trip<br />Stator winding temp high 75°C Unload >75°C Rundown trip<br />Hot gas temp high 75°C Unload >75°C Rundown trip<br />Stator core temp high 95°C Unload >105°C Rundown trip<br />Rotor winding temp high 110°C Unload >110°C Rundown trip<br />Temp of cold hydrogen gas 55°C Unload >55°C Rundown trip<br />Temp of inlet water to gas coolers 37-48°C Unload >48°C Rundown trip<br />Temp of inlet water to stator winding 44-48°C Unload >48°C Rundown trip<br />Generator seal oil outlet temp 65°C >65°C manual trip<br />Purity of hydrogen in casing <97% <95% <95% manual trip<br />*Unload – Decreasing load to a lower value manually<br />*Rundown – Reducing load to no-load condition (manually/automatic)<br />• Why boiler level high trip has been provided in turbine?<br />In condition of boiler level high moisture contents in the steam will rise and rise in<br />moisture content is harmful to turbine.<br />Question and answers Electrical Maintenance Unit<br />- 236 -<br />• What are the characteristics of protection system?<br />CHARACTERISTICS OF PROTECTIVE SYSTEM<br />Protective relaying is an important requirement in power generation, transmission and<br />distribution, which identifies the exact location of the fault and give command for<br />isolating the faulty portion very close to the fault by sensing variations in electrical<br />quantities for ensuring safe operation. The protective relay should have the following<br />characteristics:<br />a) Reliability<br />The protective relay should operate positively and isolate the faulty portion of the<br />power system as and when required.<br />b) Selectivity<br />Protection is arranged in zone, which should cover the power system completely,<br />having no part unprotected. When a fault occurs the protection is required to select<br />and trip the only the nearest circuit breaker.<br />c) Stability<br />This term, applied to protection on distinct from power network, refers to the ability of<br />the system to remain inert to all load conditions and fault external to the relevant zone.<br />d) Speed<br />The function of automatic protection is to isolate fault from the power system in a very<br />much shorter time than could be isolated manually, even with great deal of<br />supervision.<br />e) Sensitivity<br />Sensitivity is a term frequently used when referring to the minimum operating limit of<br />a complete protective system. A protective system is said to be sensitive, if the<br />primary operating current is low.<br />Question and answers Electrical Maintenance Unit<br />- 237 -<br />• What are the working principles of generator main protections?<br />GENERATOR START UP PROTECTIONS<br />SUPPLEMENTARY PROTECTION OF GENERATOR<br />The generator is normally expected to run rated speed before excitation power is applied<br />by closing the field breaker. However the residual magnetism in the field circuit may<br />provide small voltage build up even when the machine is run upto its rated speed without<br />excitation. At this stage fault if any in the generator stator circuit may not be sensed by<br />the regular protection, as must of the relays are having higher current ranges. Hence<br />separate protection (Phase &amp; Ground) are provided with low current ranges.<br />a) PHASE OVER CURRENT PROTECTION<br />The CT current is stepped down by an internal CT and converted to voltage signal. The<br />signal is compared with the internal reference. The protection is interlocked with the<br />auxiliary relay for the generator transformer breaker closed position to ensure that the<br />protection is inoperable when the machine is synchronized to grid.<br />b) GROUND FAULT PROTECTION DURING START UP<br />The generator neutral current as measure in series with the resistance of the secondary of<br />the earthing transformer is fed to the relay through CT. CT current is converted to a<br />voltage. This is compared with the internal resistance references. This protection also<br />interlocked with generator breaker position to ensure that the protection is inoperable<br />when the machine is connected to grid.<br />Question and answers Electrical Maintenance Unit<br />- 238 -<br />OTHER PROTECTIONS<br />a) STATOR EARTH FAULT PROTECTION (64A, 64B, 64C)<br />The conventional unit type generator has the neutral earthed through a resistance loaded<br />distribution type transformer. For a single ground fault near the neutral end of the<br />winding, there will be proportionately less voltage available to drive the current through<br />the ground, resulting in a lower fault current and lower neutral bus displacement voltage.<br />Low magnitude of fundamental ground current may flow under normal conditions,<br />possibly due to generator winding imbalance or due to fault on HV side of generator<br />transformer or on the secondary of generator PT. Under these conditions, the generator<br />should not be removed from service. To allow for these low magnitude earth fault<br />current, trip setting of the overvoltage ground relay are set to detect neutral displacement<br />voltage in excess of 5-10% of the phased neutral voltage.<br />If an earth fault occurs and undetected because of its location or otherwise, the probability<br />of second earth fault occurring is much greater. The second earth fault may result from<br />insulation deterioration caused by transient over voltage due to erratic, low current,<br />unstable arcing at first fault point. The second point may yield current of larger<br />magnitude.<br />A 100% stator earth fault protection is designed to detect earth fault occurring in the<br />region of the machine windings close to the neutral end. Composite static modular relay<br />that gives 100% earth fault protection of the machine, whose neutral is directly earthed. It<br />works on the principle of monitoring the neutral side and the line side of the component<br />of third harmonic voltage produced by the generator in service.<br />Question and answers Electrical Maintenance Unit<br />- 239 -<br />OPERATING PRINCIPLE<br />Alternating Current generator in service produces a certain magnitude of third harmonic<br />voltage in their winding. However no third harmonic voltage appear across the star/delta<br />connected generator, though there will be a certain magnitude of third harmonic voltage<br />between each phase and ground of the machine output. This voltage in case of machine<br />earth through high impedance can cause the flow of third harmonic current between the<br />ground and the neutral. In fact under normal healthy operating condition the third<br />harmonic voltage generated in the machine is shared between the phase to ground<br />capacity impedance at the machine terminal and neutral to ground impedance at the<br />machine neutral.<br />The figure-1 shows the third harmonic voltage distribution during normal working<br />conditions.<br />V3 = Generated third harmonic voltage.<br />VL3 = Third harmonic voltage at machine line end.<br />VN3 = Third harmonic voltage at machine neutral end<br />V3<br />VN3 VL3<br />Fig (1)<br />Whenever fault occurs at the point (Figure-2) say F on the machine winding, the voltage<br />distribution VN3 / VL3 undergoes a change from that during the running condition. In the<br />extreme case of a fault occurring on the machine neutral, the VN3 becomes zero and VL3<br />=V3. Similarly when the fault occurs on the phase terminal, VN3 become equal to V3.<br />The change in 3rd harmonic voltage will sense the relay and trip the generator.<br />N Line<br />Fault<br />V3<br />VN3<br />Faulty<br />VL3 Healthy<br />VN3 VL3 Faulty<br />Healthy<br />Question and answers Electrical Maintenance Unit<br />- 240 -<br />Fig (2) 3rd harmonic voltage distribution during healthy and faulty condition.<br />Figure-3 shows the VN3 Vs VL3 plot under healthy condition, it is clear that in order to<br />remain stable under healthy condition, the relay should restrain within the two lines L1 &amp;<br />L2. The slopes of two lines are suitably set to ensure stability.<br />Line 1<br />Fault on neutral Healthy condition<br />VL3 Line 2<br />Fault on phase<br />VN3<br />The fault scheme of main generator is having first relay 64A, covers 100% of the stator<br />winding, the 2nd relay 64B covers 0-90% of stator winding from phase terminals. The 3rd<br />relay 64C used for the protection of stator earth fault during start-up.<br />Variation of neutral and line side<br />3rd harmonic voltage at load<br />Question and answers Electrical Maintenance Unit<br />- 241 -<br />b) GENERATOR UNBALANCE PROTECTION (46)<br />Negative phase sequence current in the stator of generator due to unbalance load, fault,<br />induces double frequency eddy currents in the rotor. This current if allowed to persist,<br />can cause serious over heating. The unbalance protection relay disconnects the machine<br />before such excess over heat. In order to avoid unnecessary tripping of the machine, the<br />time characteristics of the relay should match the heating characteristics of the machine.<br />The neg. phase sequence current creates magnetic flux wave in the air gap, which induces<br />current in the rotor body iron. These currents with twice rated frequency tend to flow in<br />the non-magnetic rotor wedges and retaining rings. Heating occurs in these areas due to<br />watt loss and quickly raises the temp.<br />DESCRIPTION<br />Figure-1 shows the block diagram of the unbalance protection relay. The input from the<br />CT which are connected in the each phase of the generator supply (Fig-2) are fed to a<br />negative sequence filter (Fig-3) which gives an a.c. output voltage proportional to the<br />negative sequence current. This voltage is rectified, smoothened and fed to the squaring<br />unit of the main measuring element, the time delay circuit and the alarm unit.<br />The output of the squaring circuit is proportional to the square of the input voltage and is<br />applied directly to the main timing circuit to give the required relation ship between I2t<br />and relay operating time (t).<br />The voltage upto, which the timing capacitor charge depends upon the voltage, applied<br />from the squaring circuit. This means that even when the negative current is less than the<br />relay setting, the timer circuit will partially charges and reduces the relay operating time<br />when the current exceeds the setting value.<br />When the output exceeds the reference voltage it provides one of the input to a 2-input<br />AND gate. The other input comes from the 0.3-sec timer, which is activated by the timer<br />starter unit when the relay setting exceeds the relay setting. When the both inputs to the<br />AND gate are present the relay will operate and trip the generator from fault.<br />OPERATING PRINCIPLE<br />The negative sequence filter shown in Figure-2 is connected in delta to eliminate the<br />effect of zero sequence currents. A fourth auxiliary transformer is provided to get a phase<br />shift of 180o Ic – A in figure–3. Vector diagram of both positive and negative sequence<br />current in the filter are shown in figure-4&amp;5. It can be seen that the output produced<br />when negative sequence current is present, but zero when the current are of positive<br />sequence.<br />Question and answers Electrical Maintenance Unit<br />- 242 -<br />c) GENERATOR FIELD FAILURE PROTECTION (40)<br />Loss of field supply to a synchronous generator can be caused by a fault in the excitation<br />circuit or by incorrect opening of field breaker. On loss of field, the machine operates as<br />an induction generator excited by the reactive power drawn from the system to which it<br />connected. This could result in instability of power in the system and overheating the<br />rotor.<br />One parameter which changes significantly when the machine is subject to severe loss of<br />excitation is the impedance measured at the terminals and it move into the negative<br />reactance area. The relay is set to detect this abnormal operating condition using its<br />circular impedance characteristics, which lies in the negative reactance area.<br />OPERATION<br />Figure-1 shows the fundamental block diagram of the relay vector V and I are voltage<br />and current input to relay terminal. The input to the relay current circuit is through a CT<br />(T1), which is tapped on the both the primary, and the secondary windings to give a<br />course (K3) and medium reach (K2) setting of the relay. The relay characteristic angle is<br />continuously variable from 45o to 75o lagging by means of a potentiometer (Q). The<br />forward reach of the relay (Z) is continuously variable by means of potentiometer (K1) in<br />the voltage-restrained circuit of mixing transformer (T3).<br />Output vector S2 proportional to the vector V ± I Z of the voltage mixing transformer (T2)<br />forms the second input signal of the phase angle comparator. The comparator is a 2-input<br />block average comparator and operates by comparing the signal vector S1 &amp; S2. The<br />output of the comparator is fed into a squaring amplifier whose output switches ON for a<br />positive input and OFF for a negative input. The output waveforms of the amplifier are<br />varying mark/space square wave, mark/space being equal for 90o-phase angle difference<br />between two inputs. The squared output is averaged by an auxiliary element set to just to<br />operate for an equal mark/space ratio. The current build up in the inductive auxiliary coil<br />to reach the operate level only if the ON period are longer than the OFF period. The L/R<br />ratio of the auxiliary coil and pick up level are accurately set. The output auxiliary relay<br />then picks up if the phase angle between the signal vector S1 &amp; S2 are 90o or more as<br />shown in figure-2. Fig-3 shows the typical circuit connection for field failure protection<br />of generator.<br />Question and answers Electrical Maintenance Unit<br />- 243 -<br />d) GENERATOR POLE SLIPPING PROTECTION (78)<br />Sudden occurrence in the electrical grid such as rapid load changes, short circuit<br />interruptions, which destroy the equilibrium of the energy balance are usually followed by<br />oscillations. If the system stability is retained, the stationary stage will take over. If the<br />oscillations are not stable, a loss of synchronism of one or more machine will result. If<br />the angular displacement of the rotor exceeds the stable limit, the rotor will slip a pole<br />pitch. Pole slip occurs and excitation is maintained the machine will oscillate strongly on<br />reactive and active power side.<br />This relay operates on the principle of measuring impedance course on R-X diagram and<br />operates to trip on pole slipping condition. The scheme consists of two numbers angle<br />impedance relay and a timer to distinguish between pole slipping and power swing<br />blocking condition. When gen. Losses synchronism the resulting high current picks and<br />off freq. Operation can cause winding stresses, pulsating torque and mechanical<br />resonance that have potential of damaging the Turbine Generator.<br />X<br />Blinder Directional<br />Load Area<br />Q2 Q1 R<br />Operate Restrain<br />B Operate<br />A<br />Generator pole slipping protection<br />e) GENERATOR DIFFERENTIAL PROTECTION (87G)<br />This is a high-speed differential protection, the relay of high impedance is provided for<br />this protection. The high impedance principle is used for thorough fault stability even<br />under current transformer saturation.<br />This protection has an operating time of 25 millisecond at 5 time’s current setting. A<br />non- linear resistance is connected across the relay to limit the over voltage during<br />internal fault.<br />This protection covers phase to phase and 3-phase faults. It does not cover phase to<br />ground fault as the ground fault current is limited to a very low value. This protection<br />energizes Class-A trip.<br />Question and answers Electrical Maintenance Unit<br />- 244 -<br />Question and answers Electrical Maintenance Unit<br />- 245 -<br />f) GENERATOR INTER TURN DIFFERENTIAL PROTECTION: (87 GI)<br />This protection is by means of a differential current relay connected across crossconnected<br />CT on the two parallel winding of each of the phase of the generator as shown<br />in figure-2. The relay which is used for t he protection is of high impedance circulating<br />current type with an operating speed of 25 millisecond at 5 times the current setting. A<br />non-linear resistance is connected across the relay to limit the over voltage during the<br />internal fault. This protection energizes Class-A trip.<br />PRINCIPLE OF OPERATION (DIFFERENTIAL)<br />Fig-3 shows the simplified diagram of differential current protection of generator<br />winding, the CT’s of both end of the generator winding will sense the current which is<br />flowing through the stator winding. During normal balanced condition the current vector<br />I1 &amp; I2 are equal and opposite so the resultant forces experiences in the coil of the relay R<br />is zero.<br />When the fault ‘F’ occurs on the stator winding, the differential current will be sensed by<br />the CT and these differential current passes through the operating coil of the relay which<br />gives trip signal to the circuit breaker of the generator.<br />Ground To load<br />Fault<br />I1 I2<br />I1 I3 I2<br />I1 + I2 = 0 Normal condition<br />I1 + I2 = I3 Faulty condition<br />Question and answers Electrical Maintenance Unit<br />- 246 -<br />GENERATOR BACK UP PROTECTIONS<br />a) UNDER FREQUENCY PROTECTION (81)<br />The U/F limitations however are less restrictive than the limitations on the turbine. A<br />turbine blade is designed to have its natural frequencies sufficiently displaced from rated<br />speed and multiples of N (speed) to avoid a mechanical resonant condition that could<br />result in excessive mechanical Stresses in blades<br />This is a three stage under frequency protection, which consists of a time delay unit and 3<br />timer. The three stages of frequencies are ranging from 47 to 50 Hz. The timer which<br />gives the cumulative operating time of turbine during under frequency which calls for<br />turbine inspection/maintenance as per the design formula.<br />(48.5-F) t < 3.<br />Where F is the frequency,<br />t is the timer duration in seconds.<br />From the above formula, it can be seen that the turbine can be operable at 48.5 Hz<br />continuously at rated load. The cumulative timer which gives alarm in Data acquisition<br />system then call for turbine inspection.<br />OPERATING PRINCIPLE:<br />The operating principle of the relay is the comparison of the incoming frequency with that<br />of a pre-set value of time derived from the oscillator of the relay.<br />The incoming frequency signal is connected to an input circuit, which then drive an<br />impulse generator to produce pulse at the beginning of each period of the input voltage.<br />The preset time interval is obtained from an oscillator and counter, adjustment is achieved<br />using selector switches, which drives the decoder circuit.<br />A comparator compares the two-time interval and this triggers an adjustable timer, which<br />then operate the output voltage. An under voltage detector inhibits the relay when the<br />incoming signal drops below the preset value.<br />b) OVER FREQUENCY PROTECTION (81)<br />Generator over frequency protection is provided to limit the over speeding of turbine,<br />which leads to greater vibration due to resonance. The over speeding and vibration leads<br />to mechanical damage of turbine bearings and blades. This protection schemes also<br />similar to under frequency. The preset time of over frequency operation is more than the<br />preset time of under frequency protection.<br />c) GENERATOR OVER VOLTAGE ALARM (59)<br />This protection give time delayed alarm for continuous operation of the generator at more than<br />permissible voltage of AVR failure or during manual control of excitation.<br />d) GENERATOR ANTIMOTORING PROTECTION (32)<br />Motoring results from low prime mover input to generator. While generator is still in line. When this<br />input is less than no load losses deficiency is supplied by absorbing real power from the system. Since<br />the field excitation should remain same, The same reactive power would flow as before the motoring and<br />generator will operate as a synchronous motor driving the turbine. Generator will not be harmed by this<br />action but turbine can be harmed through over heating. It is detected by low forward power relay.<br />Question and answers Electrical Maintenance Unit<br />- 247 -<br />EXCITATION SYSTEM PROTECTIONS<br />The generator is provided with static excitation, which obtains the necessary excitation<br />power from the excitation transformer, which rectifies and feed the AC power through<br />controlled rectifier circuits.<br />a) EXCITATION TRANSFORMER OVER CURRENT PROTECTION:<br />Time delayed over current protection with instantaneous high set unit is provided for the<br />short circuit protection of the excitation transformer, which trips the field breaker by<br />energizing class-B trip.<br />b) ROTOR OVER VOLTAGE PROTECTION:<br />This protection is envisaged to limit over voltage occurring in the field circuit during<br />excitation of the field an air gap arrestor with a series resistor is connected across the<br />field. On overvoltage the gap flasher over and the arrestor connects the resistor directly<br />across the field.<br />This over voltage is not due to the field forcing. Field forcing will happen only when PT<br />actual voltage value comes down due to the PT fuse drop or due to any other reason. At<br />that time PT voltage is 110 V – drop. That is actual voltage value is less and field forced<br />to increase the voltage. Field forcing value is twice the actual value after looking the<br />system healthiness. Means in some earth faults in the grid, the voltages may come down<br />to 110 kV and PT will sense this voltage as the generator is synchronised with the grid.<br />This will force the field of the generator to match the generator actual voltage. If the fault<br />not cleared the generator will trip after some time delay. This is generator field forcing.<br />But in some grid disturbances or power swing conditions the stator and rotor voltage and<br />current changes. This will induce some voltage in rotor. This protection is used to protect<br />machine from this type of over voltage.<br />c) ROTOR 1ST EARTH FAULT PROTECTION<br />A single earth fault is not in itself dangerous since it does not cause fault current, but a<br />second earth fault effectively short circuits parts or all of the field system and the<br />unbalancing of the magnetic forces causes. That force may be sufficient to spring the<br />shaft and make it eccentric. If the condition were allowed to persist, however it might<br />lead to severe mechanical damage.<br />The method of detecting rotor first earth fault using the principle of negative biasing,<br />where by an earth fault anywhere in the field circuit can be detected. The dc injection<br />supply establishes a small bias on the alternator field circuit so that all points are negative<br />with respect to earth.<br />The rectified output of the supply provides a biasing potential of approximately 65V.<br />This is connected with a positive terminal to earth and negative terminal to the positive<br />terminal of the field circuit through a relay. When the fault occurs, the current flows<br />through the relay coil which intern operate the circuit breaker. This relay will not operate<br />Question and answers Electrical Maintenance Unit<br />- 248 -<br />on auxiliary supply failed condition, during that time the relay will give annunciation in<br />main control room.<br />Question and answers Electrical Maintenance Unit<br />- 249 -<br />d) ROTOR 2ND EARTH FAULT (64F)<br />While the machine is continuous in service with one earth fault, appearance of 2nd earth<br />fault will severely affect the magnetic balance in the air gap and result in rotor distortion<br />and severe damage. Hence it is advisable that the machine taken out of service as early as<br />possible after appearance of 1st earth fault. However, to take care of the situation of 2nd<br />earth fault appearing immediately after 1st stator earth fault before the machine is taken<br />out, 2nd rotor earth fault protection is provided. This protection system normally<br />disconnect the field effect and has top be switched ON when 1st earth fault appears.<br />The scheme consists of a bridge circuit which to be balanced manually with the 1st rotor<br />earth fault in the machine. This balance is disturbed when the 2nd earth fault appears and<br />the bridge null deflector initiate tripping of the circuit.<br />It can be seen in the below diagram the protection of the field winding on either side of<br />the first earth fault and the balancing potentiometer forms a dc bridge with 64F2 (Relay)<br />connected across the pair of opposite modes.<br />64F2<br />1st E/F Balancing potentiometer<br />Field 2nd E/F Excitation supply<br />E Fig (1)<br />Fig (1)<br />Discharge resistor -ve<br />-ve Field winding<br />Excitation supply<br />+ve +ve<br />64 F1 Trip and alarm<br />Relay<br />Current limiting resistor<br />AC<br />Supply<br />Fig (2)<br />Question and answers Electrical Maintenance Unit<br />- 250 -<br />Question and answers Electrical Maintenance Unit<br />- 251 -<br />TRANSFORMER PROTECTIONS<br />a) OVERALL DIFFERENTIAL PROTECTION (87 GT)<br />This protection which is used as the differential protection of the transformer, also covers<br />the generator and unit transformer. The differential transformer protection measuring<br />circuit is based on the well-known MERZ-PRICE circulating current principle.<br />Fig-1 shows the relay functional block diagram. The output from each bias resistance<br />transformer T3 to T5 proportional to the appropriate primary line currents, are rectified<br />and summed to produce a bias resistance voltage. Any resulting difference current is<br />circulated through the transformer T1 &amp; T2. The output from T1 is rectified and combined<br />with the bias voltage to produce a signal, which is applied to the amplitude comparator.<br />The comparator output is in the form of pulses which vary in width depending on the<br />amplitude of the combined bias and difference voltages where the measurement of the<br />interval between these pulse indicate less than a preset time, an internal fault is indicated<br />and a trip signal initiated after a short time delay (1/f sec), level set by the bias.<br />An unrestrained high set circuit, which monitors the differential current, will over ride the<br />amplitude comparator circuit and operate the relay output element when the difference<br />current is above the high set settings.<br />Fig-2 shows the basic circuit diagram of the differential protection and fig-3 shows the<br />current direction of the restraint/differential transformers in the relay. The currents I1, I2,<br />&amp; I3 are the output of generator CT, UT CT and GT CT respectively. These currents is<br />passing through the star connected restraint transformer, the algebraic sum of vector<br />(I1+I2+I3 = I4) is passing through the differential transformer, which will give the output<br />for operating the relay (87).<br />b) OVER FLUXING PROTECTION (59V/F)<br />This is designed to protect the transformer from damages caused by the flux density in the<br />core exceeds the designed value. The excessive flux can cause serious overheating of<br />metallic parts and in extreme case can cause localized rapid melting of generator and<br />transformer core laminations. Over fluxing can be caused by regulator failure, load<br />reduction or excessive excitation with generator off-line it can also result from decreasing<br />speed while the regulator or the operator attempts to maintain rated stator voltage. Its<br />main application is to protect the transformers where, unless considerable care is taken,<br />the flux density can become excessive during the running up or running down sequence.<br />The flux density in the core depends on the ratio of terminal voltage (V) divided by the<br />frequency (f). Normally the over fluxing withstand characteristics of the transformer are<br />120% over fluxing for 2 minutes<br />135% over fluxing for 1 minutes<br />140% over fluxing for 5 seconds.<br />Whenever the v/f ratio of the transformer exceeds the pre-set time, the relay will operate<br />and initiate<br />• Running down the AVR if the machine is off the bus bar.<br />• Tripping the GT breaker.<br />Question and answers Electrical Maintenance Unit<br />- 252 -<br />Question and answers Electrical Maintenance Unit<br />- 253 -<br />OPERATING PRINCIPLE:<br />The basic principle of the relay is to produce an alternating voltage, which is proportional<br />to the ratio of voltage &amp; frequency, and to compare this with a fixed voltage. When the<br />peak of the alternating voltage exceeds the fixed dc reference, the first timer is started. At<br />the end of the fixed timer cycle the second adjustable timer is initiated.<br />To obtain the correct measuring quantity the applied voltage V is converted to a current<br />by means of a resistor R. This V/R is arranged to flow through a capacitor C to produce<br />an output voltage<br />V/2 π f RC.<br />Over fluxing relay which consists of Voltage/Frequency measuring circuit, which output<br />is given to a comparator, compares with dc reference and to give an output after a fixed<br />time delay of 0.5 to 1.0 seconds. After the end of fixed time delay, the 2nd variable timer<br />initiates. The fixed time auxiliary has one of its two pairs of contact wired out which is<br />normally arranged to operate a follower.<br />c) GENERATOR TRANSFORMER RESTRICTED EARTH FAULT PROTECTION (64)<br />In addition to overall differential protection, a restricted earth fault protection covering<br />the transformer HV winding only is provided. The zone of protection extends from CT<br />provided on the transformer neutral end to the CT provided on the transformer bushings.<br />The relay is high impedance type and high speed of operation. A non-linear resistance is<br />connected across the relay terminal to limit the voltage developed during serial internal<br />fault. This protection energizes Class-A trip of the turbo-generator.<br />REF relay<br />Transformer<br />SCHEME OF RESTRICTED E/F PROTECTION<br />d) GENERATOR TRANSFORMER BACK-UP OVER CURRENT PROTECTION FOR PHASE FAULT (51)<br />This protection consists of a 3 phase over current relay. The relay is 3-pole version of<br />very inverse time over current relay plus high set instantaneous over current relay. This<br />will act as the back up protection for the transformer fault due to the fault current flowing<br />from system side. This may also serve limited back up protection function for fault<br />external to the transformer. This will energize Class-A trip.<br />R<br />Question and answers Electrical Maintenance Unit<br />- 254 -<br />e) GENERATOR TRANSFORMER BACK UP EARTH FAULT PROTECTION (51N)<br />This is a simple inverse type over current relay connected to the neutral CT of<br />transformer. This relay provided back up function for fault both internal and external to<br />the transformer, This protection energizes Class-A trip.<br />Question and answers Electrical Maintenance Unit<br />- 255 -<br />f) OVER LOAD MONITORING (49)<br />Measuring oil temperature and winding temperature indirectly monitors the loading of the<br />generator transformer. The oil and winding temperature indicators are provided with<br />contacts for initiating alarms as a first stage and tripping as the second stage whenever the<br />oil and winding temperature limits are exceeded. The oil temperature /winding<br />temperature trips are routed through Class-C trip.<br />g) GAS PROTECTION (63)<br />A Buchholz relay is supplied along with the transformer. The relay has two contacts one<br />closes on slow gas formation and initiate alarm. The second contact closes of sudden<br />surge of oil flow in case of severe internal fault and this contact is wired for tripping the<br />unit in Class-A trip.<br />The relay consists of two float switches contained in a closed housing, which is located in<br />the pipe from transformer to conservator tank. Any internal fault in the transformer<br />comes, the oil decomposes and the generating gases which passes up the pipe towards the<br />conservator and is trapped in the relay. In this two float relay the top float responds the<br />slow accumulation of gas due to mild and incipient fault, the lower float being deflected<br />by the oil surges caused by a major fault. The float control contacts, in the first stage give<br />an alarm and second case to isolate the transformer.<br />Question and answers Electrical Maintenance Unit<br />- 256 -<br />• What are the set values of generator protection?<br />TYPE OF PROTECTION AND ITS SET VALUES<br />No. Type of Protection Set Values CT/PT Ratio Time Delay Class<br />1. Generator differential 0.5A(10%) 10000/5 Inst. Class-A1<br />2. Generator Inter turn 0.5A(10%) 5000/5 Inst. Class-A1<br />3 Generator reverse power 0.5% 10000/5 5 sec Class-A1<br />(stage 2 Tx trip)<br />4. 100% Stator Earth Fault ND = 5V(3r<br />harmonic 70<br />100%)<br />16.5 kV/110V 1.0 sec Class-A1<br />5. 2nd Rotor Earth Fault 1.0 mA --- --- Class-A1<br />6. Over Frequency 51.5 Hz 16.5 kV/110V 0.1 sec 86 BG<br />7. Over Voltage 120% 16.5 kV/110V 2.0 sec Class-A<br />8. Overall Differential 1.0A 10000/5A Inst. Class-A1<br />9. GT Restricted E/F 0.1A 800/1A Inst. Class-A1<br />10. GT Gas Protection --- ---- Inst. Class-A1<br />11. GT Fire --- --- Inst. Class-A1<br />12. GT Over Current PSM-1.0<br />Inst. – 8.0<br />800/1 A TMS=0.4 Class-A1<br />13. GT Earth Fault PSM-0.2<br />TMS-0.52<br />800/1A 2.0 sec Class-A1<br />14. Impedance Protection<br />Stage-1<br />--- 10000/5A 2.0 sec Class-A2<br />15. Generator Over Current<br />During Starting<br />50 mA 10000/5A Inst. Class-A2<br />16. Generator Back-up Earth<br />Fault<br />PSM-5.4V 16.5 kV/110V TMS =0.3 Class-A2<br />17. Stator Earth Fault During Starting 100 mA 300/1A Inst. Class-A2<br />18. Low Forward Power 0.5% of FP 10000/5A 2.5 sec Class-B1<br />19. GT Over Fluxing Stage-1 120% --- 2 min Class-B<br />20. GT Over Fluxing Stage-2 135% --- 1 min Class-B<br />21. Negative Sequence 5% 10000/5A Inverse Class-B<br />22. Field Failure --- 10000/5A Inverse Class-A2<br />23. Under Frequency 47.77 Hz 16.5 kV/110V 4 sec Class-C<br />24. GT Winding Temp. High 130O C --- --- Class-C<br />25. GT Oil Temp. High 90O C --- --- Class-C<br />• What is arc and what is spark?<br />Spark - the heat produced that ignites, due to the rubbing of two metals is called the<br />spark.<br />Question and answers Electrical Maintenance Unit<br />- 257 -<br />Arc – the electrical discharge between two electrodes is called the arc. Arc is the<br />self-sustained discharge of electricity between electrodes in a gas or vapour, which<br />has a high voltage discharge at the cathode.<br />Question and answers Electrical Maintenance Unit<br />- 258 -<br />• What precautions should be taken while meggering main generator?<br />All PT’s are racked out.<br />Earthing transformer grounding terminals disconnected.<br />Barring gear shall be kept off.<br />Stator water shall be drained fully and hot air blown through conductors.<br />Generator flexible lines shall be disconnected to isolate GT/UT.<br />• What is the speed equation for AC machine?<br />N = 120 f / P<br />N – Speed in RPM<br />f - Frequency in Hz<br />P – Number of poles<br />• What is emf equation of alternator?<br />Emf = 4.44 kc kd f ∅ T volts.<br />Kd = Distribution factor = sin m β/2<br />m sin β/2<br />kc/kp = Coil span factor /Pitch factor = cos α/2<br />kf = Form factor = 1.11<br />∴Average emf induced / Cycle = ∅ N P/ 60<br />= ∅ P ∗120 f<br />60 * P<br />=2 f ∅ volt<br />If Z is the number of conductors = 2T (T = two sides of conductor)<br />emf induced = 2 f ∅ Z =2 f ∅ 2T = 4 f ∅ T<br />∴ RMS value of emf induced = form factor * emf<br />= 1.11 * 4 f ∅ T<br />= 4.44 f ∅ T volts.<br />• What is the emf equation for DC generator?<br />P * ∅ * Z * N<br />60 * A<br />A = number of parallel paths. That is for lap winding it is equal to Z and for wave<br />winding it equal to 2.<br />• What are the factors, which varies terminal voltage of generator?<br />a) Voltage drop due to resistance (Ra drop). This is negligible.<br />b) Voltage drops due to leakage reactance (XL).<br />c) Voltage drops due to armature reaction.<br />Question and answers Electrical Maintenance Unit<br />- 259 -<br />• What is meant by Armature reaction?<br />The effect of armature flux on the main field flux is called Armature reaction, where<br />armature flux weakens the main field flux. In Alternator power factor contributes<br />more importance in Armature reaction.<br />a) In Unity power factor field strength is average and effect is distortional. So<br />voltage variation will not be too much.<br />b) In lagging power factor armature flux is directly opposite to the main field flux.<br />That is armature flux is lagging 90ο by main field flux. So the result is<br />demagnetizing the field. Due to less field flux less voltage at the alternator<br />terminals and excitation required is more.<br />c) In leading power factor armature flux is leading by 90ο to the main field flux. The<br />result is additive and main field strength is more and excitation has to be reduced.<br />Otherwise end parts or overhang portion of the generator will heat.<br />• What is meant by voltage regulation?<br />If there is a change in load, there is a change in terminal voltage. This change not<br />only depends upon the load but also on power factor. The voltage regulation is<br />termed as the rise in voltage when full load is removed divided by rated terminal<br />voltage (Excitation and speed remains constant).<br />∴ Regulation in % = E0 – V<br />V<br />In case of leading power factor terminal voltage will fall and regulation is negative.<br />PF leading<br />Terminal<br />Voltage PF unity<br />PF lagging<br />Load current<br />Generator voltage characteristics<br />• Why double squirrel cage motor used in barring gear? Why?<br />To have high starting torque.<br />In AC motors torque is directly proportional to φ (flux), I2 and cosφ2.<br />i.e T ;φ (flux* I2 * cos φ2.<br />∴ T = k * φ (flux)* I2 * cos φ2.<br />Rotor at standstill E2;φ (flux)<br />∴ T = k * E2 * I2 * cos φ2.<br />Question and answers Electrical Maintenance Unit<br />- 260 -<br />In double squirrel cage motor inner cage is low resistive and high inductive. The<br />outer cage is high resistive and low inductive. In case of inner cage Z (impedance) is<br />less (XL = 2􀀟 f L). If the rotor is having high inductance at starting I2 will lag E2 by<br />large and cos φ2 (Rotor PF = R2 / Z2) is very less. So torque<br />is less.<br />At staring rotor torque is proportional to the rotor<br />resistance. At starting inductance is high and the Z is--<br />--------<br />Question and answers Electrical Maintenance Unit<br />- 261 -<br />• What are the logics adopted to close the field breaker?<br />a) Turbine speed 2880 rpm.<br />b) Class A, B and BG trip reset.<br />c) Auto/manual reference minimum.<br />d) Auto/manual channels supply normal.<br />e) FB closing circuit healthy.<br />• What you mean by positive sequence, negative sequence and zero sequence of<br />voltage?<br />Positive phase sequence<br />A system of vectors is said to have positive sequence if they are all of equal<br />magnitude and are displaced by 120° with same time interval to arrive at fixed axis<br />of reference as that of generated voltage. The positive phase sequence is represented<br />below and the vectors arrive along X-axis in order 1, 2, 3 and conscript P has been<br />used to designate as positive sequence.<br />E3P<br />Anti clock direction<br />120°<br />E3P<br />E3P<br />Negative phase sequence<br />A system of vectors is said to have a negative phase sequence if they are of equal<br />magnitude displaced at an angle of 120° but arrive at the axis of reference at the<br />regular interval same as that of positive phase sequence but in order of 1, 3, 2. That<br />is the order is reversed.<br />E3N<br />Clock direction<br />120°<br />E3N<br />E3N<br />Zero sequence<br />A system of vectors in a phase system is said to have zero phase sequence if all the<br />three vectors are not displaced from each other and there will be no phase sequence<br />Question and answers Electrical Maintenance Unit<br />- 262 -<br />in such cases. The current or voltages in the 3-phase circuit vary simultaneously in<br />all the 3- phases. Such phase sequence is zero phase sequence.<br />E1O<br />E2O<br />E3O<br />Question and answers Electrical Maintenance Unit<br />- 263 -<br />• What is rotor and stator resistance values?<br />Rotor resistance = 98.1 m􀀍<br />Stator winding resistance’s<br />R φ = 3.1􀀍/3.1􀀍<br />Y φ = 3.1􀀍/3.1􀀍<br />B φ = 3.1􀀍/3.1􀀍<br />• What is the rating of generator PT fuse?<br />24 kV, 3.15 Amps.<br />• What is the wearing rate of generator Slipring?<br />Generator Slipring wearing rate is 0.025 mm /1000 hrs.<br />• What is the brush pressure on Slipring?<br />Recommended brush pressure in the Slipring is 150 to 200 gms/cm2 (0.9 to 1 kg).<br />• What are the properties of hydrogen and DM water?<br />Hydrogen<br />a. Windage losses are less. Hence efficiency increased.<br />b. Heat transfer is more. Hence output per volume is increased.<br />c. No corona discharge, which makes insulation life long.<br />d. Lesser denser and penetration and cooling more.<br />e. No fire risk at purity 4% to 74%.<br />DM Water<br />a. Non toxic and low viscosity.<br />b. High thermal conductivity.<br />c. Low conductivity.<br />d. Freedom from fire risk.<br />e. External heat exchanger used.<br />• What are the chemical tests on hydrogen and DM water?<br />Hydrogen<br />a) Hydrogen purity in % (volume/volume).<br />b) Relative humidity in % (30% is nominal).<br />DM water<br />a) PH of DM water (less than 6.5 is acidic and more than 7 is alkaline where oxygen<br />is not forming). PH is also called IP (isotopic purity).<br />b) Conductivity.<br />c) Copper traces.<br />d) Dissolved oxygen (to trace corrosion and 1.2% is more).<br />Question and answers Electrical Maintenance Unit<br />- 264 -<br />Question and answers Electrical Maintenance Unit<br />- 265 -<br />• What are the logic’s adopted in barring gear motor?<br />For start permission<br />a) Local or remote start.<br />b) JOP is running.<br />c) Motor hand barring is permissive.<br />d) 42 contactor in MCC is off.<br />e) Turbine speed is <100 rpm.<br />Start permission (42S of MCC)<br />a) All above<br />b) Bearing oil pressure is >0.35 kg/ cm2.<br />c) No thermal over load of 42S.<br />d) No one-DG condition.<br />Start permission (42 of MCC)<br />a) Start permissive of 42S.<br />b) Barring gear engage or motor speed reached to 1475 rpm.<br />c) Turbine speed is <100 rpm.<br />d) Bearing oil pressure is >0.35 kg/ cm2.<br />e) No thermal over load of 42.<br />f) No one-DG condition.<br />• What is the equation for resistance measurement of PT 100 thermocouple?<br />°C = (R-100) / 0.39<br />• What are the requirements for synchronization and setting for generator?<br />a) Same phase sequence.<br />b) Voltage should in-phase and angle should not be more than 10°.<br />c) Voltage value must be same and difference of 5% is allowed.<br />d) Frequency should be same and difference of 0.1% i.e. 0.05 Hz is allowed.<br />• What is the recommended IR value for generator?<br />Main generator is class B insulated machine. Without stator water recommended<br />insulation value for the generator is R60 = kV + 1 MΩ<br />R60 – minimum recommended IR in MΩ of entire winding at 40°C of 60 Sec.<br />kV – rated voltage.<br />For the IR measurement 1 kV megger should be used.<br />• What is the type turbine installed in KGS?<br />Tandem compounded to expansion of steam, impulse reaction type.<br />• State HP &amp; LP turbine steam values.<br />HP LP<br />Question and answers Electrical Maintenance Unit<br />- 266 -<br />Pressure Flow Temp Pressure Flow Temp<br />I/L 40 kg/cm2 1333 t/h 250°C I/L 5.664 kg/cm2 232.9°C<br />O/L 6.02 kg/cm2 O/L<br />Wetness (I/L) 0.26% Wetness (I/L)<br />Wetness (O/L) 11.058% Wetness (O/L)<br />Question and answers Electrical Maintenance Unit<br />- 267 -<br />• State turbine governor setting.<br />On 2560 rpm turbine governor becomes effective and on 2760 rpm is turbine<br />governor take over speed.<br />• What is requirement of speeder gear assembly?<br />To bring the turbine to synchronous speed and get tight lock with grid by grid<br />frequency. BPC signal is given in Auto mode to the speeder gear motor.<br />• What is the purpose of LLG?<br />To ensure that the turbine load never exceed the reactor output, to incorporate<br />turbine follow reactor feature governing system.<br />• What is the purpose of OSLG?<br />This gear mainly used to control the steam flow so as to limit the machine from over<br />speeding. On following occasions the over speed limiting gear acts.<br />a) When the flow of steam corresponds to load is 2/3 and<br />b) Electrical power on generator falls 1/3 of full load.<br />• What is the logic in lubrication oil pump system?<br />Normally main oil pump (MOP) will feed the required lub oil to turbine governor<br />and lubrication. If the pressure drops to 5.3 kg/cm2 6.6 kV 373 kW Aux. Oil Pump<br />will start. If further pressure falls to <0.65 kg/cm2 Flushing Oil Pump will start. If<br />further pressure drops <0.35 kg/cm2 Emergency Oil Pump will start.<br />Lubricating oil inlet temperature will be 40°C and outlet temperature will be 70°C.<br />• What is the purpose of TOPP (turbine oil purification plant)?<br />The purpose of TOPP is to remove the water ingress in turbine oil system from the<br />gland leaks, cooler leakage, and solid metal particles, which are produced due to rust,<br />wear of bearings and to normalize the low quality oil.<br />The remove capacity of TOPP is, for solids – 5 microns and for water – 300 to 500<br />parts per milli.<br />Question and answers Electrical Maintenance Unit<br />- 268 -<br />RELAYS<br />• Write down the relay numbers and their designation.<br />1 MASTER ELEMENT 51 AC TIME OVER CURRENT RELAY<br />2 TIME DELAY STARTING OR CLEARING 52 AC CIRCUIT BREAKER<br />3 CHECK OR INTERPOSING RELAY 53 EXCITER OR DC GENERATOR<br />4 MASTER CONTACTOR 54 SPARE<br />5 STOPPING DEVICE 55 POWER FACTOR RELAY<br />6 STARTING CIRCUIT BREAKER 56 FIELD APPLICATION RELAY<br />7 ANODE CIRCUIT BREAKER 57 SHORT CIRCUITING DEVICE<br />8 CONTROL POWER DISCONNECT DEVICE 58 RECTIFICATION FAILURE RELAY<br />9 REVERSING DEVICE 59 OVER VOLTAGE RELAY<br />10 UNIT SEQUENCE RELAY 60 VOLTAGE OR CURRENT BALANCE RELAY<br />11 SPARE 61 SPARE<br />12 OVER SPEED RELAY 62 TIME DELAY STOPPING OR OPENING DEVICE<br />13 SYNCHRONISING SPEED DEVICE 63 LIQUID OR GAS OR VACCUM RELAY<br />14 UNDER SPEED DEVICE 64 GROUND PROTECTION RELAY<br />15 SPEED OR FREQUENCY MATCHING DEVICE 65 GOVERNOR<br />16 SPARE 66 NOTCHING OR JOGGING RELAY<br />17 SHUNTING OR DISCHARGE SWITCH 67 AC DIRECTIONAL OVER CURRENT RELAY<br />18 ACCELERATING OR DE-ACCELERATING DEVICE 68 BLOCKING RELAY<br />19 STARTING OR RUNNING TRANSITION DEVICE 69 PERMISSIVE CONTACT DEVICE<br />20 ELECTRICALLY OPERATED VALVE 70 RHEOSTAT, ELECTRICALLY OPERATED<br />21 DISTANCE PROTECTION RELAY 71 LIQUID OR GAS LEVEL RELAY<br />22 EQUALIZER CIRCUIT BREAKER 72 DC CIRCUIT BREAKER<br />23 TEMPERATURE CONTROL DEVICE 73 LOAD RESISTOR CONTACTOR<br />24 SPARE 74 ALARM RELAY<br />25 SYNCHRONISING DEVICE 75 POSITION MECHANISM<br />26 APPARATUS THERMAL DEVICE 76 DC OVER CURRENT RELAY<br />27 UNDER VOLTAGE RELAY 77 PULSE TRANSMITTER<br />28 FLAME DETECTOR 78 PHASE ANGLE OR OUT OF STEP RELAY<br />29 ISOLATING CONTACTOR 79 AC RECLOSING RELAY<br />30 ANNUNCIATER RELAY 80 SUPPLY FAIL<br />31 SEPARATE EXCITATION DEVICE 81 FREQUENCY RELAY<br />32 DIRECTIONAL POWER RELAY 82 DC RECLOSING RELAY<br />33 POSITION SWITCH 83 AUTOMATIC SELECTION<br />34 MASTER SEQUENCE DEVICE 84 OPERATING MECHANISM<br />35 SLIP RING SHORT CIRCUIT DEVICE 85 CARRIER OR PILOT WIRE RECEIVER RELAY<br />36 POLARITY OR POLARIZING VOLTAGE DEVICE 86 LOCK OUT RELAY<br />37 UNDER CURRENT OR UNDER POWER RELAY 87 DIFFERENTIAL PROTECTION RELAY<br />38 BEARING PROTECTIVE DEVICE 88 AUXILIARY MOTOR OR MOTOR GENERATOR<br />39 MECHANICAL CONDITION MONITOR 89 LINE SWITCH<br />40 FIELD RELAY 90 REGULATING DEVICE<br />41 FIELD CIRCUIT BREAKER 91 VOLTAGE DIRECTIONAL RELAY<br />42 RUNNING CIRCUIT BREAKER 92 VOLTAGE &amp; POWER DIRECTIONAL RELAY<br />43 MANUAL TRANSFER OR SELECTOR DEVICE 93 FIELD CHANGING RELAY<br />44 UNIT SEQUENCE STARTING RELAY 94 TRIPPING OR TRIP FREE RELAY<br />45 ATMOSPHERIC CONDITION MONITOR 95 SUPERVISION RELAY<br />46 CURRENT UNBALANCE RELAY 96 SPECIAL APPLICATION<br />47 POLE DISCREPANCY 97 FUSE FAIL RELAY<br />48 INCOMPLETE SEQUENCE RELAY 98 SPECIAL APPLICATION<br />49 THERMAL OVER LOAD RELAY 99 OVER FLUXING RELAY<br />50 INSTANTANEOUS OVER CURRENT RELAY 100 SPECIAL APPLICATION<br />Question and answers Electrical Maintenance Unit<br />- 269 -<br />General Description of Relays<br />NOMENCLATURE FOR ENGLISH ELECTRIC RELAY<br />FIRST LETTER – OPERATING QUANTITY<br />A PHASE ANGLE COMPARISON SECOND LETTER – MOVEMENT<br />B BALANCED CURRENT A ATTRACTED ARMATURE<br />C CURRENT B BUCHHOLZ<br />D DIFFERENTIAL C INDUCTION CUP<br />E DIRECTION D INDUCTION DISC<br />F FREQUENCY G GALVANOMETER (MOVING COIL)<br />I DIRECTIONAL CURRENT T TRANSISTOR<br />K RATE OF RISE OF CURRENT<br />N MANUAL<br />O OIL PRESSURE<br />P POLY PHASE VA<br />R REACTIVE VA<br />S SLIP FREQUENCY<br />T TEMPERATURE<br />V POTENTIAL<br />W WATTS (POWER)<br />Y ADMITTANCE<br />Z IMPEDANCE<br />THIRD LETTER – APPLICATION<br />A AUXILIARY R RE CLOSING<br />B TESTING S SYNCHRONISING<br />C CARRIER (COUNTING) T TIMER OR TRANSFORMER<br />D DIRECTIONAL U DEFINITE TIME<br />E EARTH (GROUND) V VOLTAGE TIME<br />F FLAG &amp; ALARM INDICATOR W PILOT WIRE<br />G GENERAL OR GENERATOR WA INTERPOSING<br />H HARMONIC RESTRAINT WJ INTER TRIPPING<br />I INTERLOCK OR INDUSTRIAL X SUPERVISORY<br />J TRIPPING Y FLASH BACK (BACK FIRE)<br />JE TRIPPING (ELECT. RESET) Z SPECIAL APPLICATION<br />JH TRIPPING (HAND RESET) ZS ZERO SEQUENCE<br />JS TRIPPING (SELF RESET)<br />JC CONTROL<br />K CHECK ALARM<br />L LIMITING<br />M SEMAPHORE OR MOTOR<br />N NEGATIVE SEQUENCE<br />O OUT OF STEP<br />P POTENTIAL FAILURE<br />Q ALARM<br />FOURTH LETTER<br />M – SPECIAL VARIATION<br />Sl. No. E E Relay Application<br />1 CTM Motor protection<br />2 CTU Locked rotor. Thermal alarm<br />3 CDG I.D.M.T. over current or earth fault of transformer<br />4 CAG Instantaneous over current or earth faults.<br />Question and answers Electrical Maintenance Unit<br />- 270 -<br />5 VAGM Under voltage<br />6 WDG Under /Over power for DG set<br />7 FTG Under frequency<br />8 VAPM Fuse failure<br />Question and answers Electrical Maintenance Unit<br />- 271 -<br />• What is Knee point voltage?<br />EMF applied to secondary of current transformer (CT) which, when increased by<br />10% voltage causes the excitation current to increase by 50%.<br />• What is I.D.M.T?<br />Inverse time relay with definite minimum time is called IDMT.<br />• What is Negative sequence reactance?<br />Negative sequence can arise whenever there is any unbalance present in the system.<br />Their effect is to setup a field rotating in opposite direction to the main field.<br />• What is Zero sequence reactance?<br />If a machine is operating with an earthed neutral, a system earth fault will give rise to<br />zero sequence current in the machine.<br />• Purpose of over current relay (Inverse); type- CDG<br />It is a self powered inverse time over current and earth fault relay, used for selective<br />phase and earth fault protection in time graded systems for A.C. machines,<br />transformers, feeders etc. A non-directional heavily damped induction disc relay,<br />which has an adjustable inverse time/current characteristic with a definite minimum<br />time. The relay has a high torque movement combined with low burden and low<br />overshoot. The relay disc is so shaped that as it rotates the driving torque increases<br />and offsets the changing restraining torque of the control spring.<br />• Purpose of Directional inverse Over current &amp; earth fault relay; type- CDD<br />Directional phase or earth fault protection of ring-mains, parallel transformers,<br />transformer feeders, parallel feeders etc., employing the time graded principle.; This<br />relay comprises an inductive disc over current unit with wound shading coils and a<br />directional high speed induction cup unit. The cup-unit contact is wired across the<br />shading coils so that no torque is exerted on the disc of the over current unit until the<br />cup unit contact closes. The inductive disc unit is thus directionally controlled and it<br />operates only when the current flows in the tripping direction. The directional unit is<br />a high speed, low inertia four pole induction cup movement designed to give a high,<br />steady and non-vibrating torque. its current coil is connected in series with the<br />operating coil of the induction disc unit. The directional unit is normally provided<br />with voltage polarising coils.<br />• Purpose of Over current &amp; earth fault relay; type- CAG<br />This relays are designed for instantaneous phase or earth fault protection and<br />instantaneous high set over current protection.; A standard hinged-armature unit<br />forms the basic movement for this relay. It consists an operating coil mounted on a<br />cylindrical iron core bolted to a frame at one end. This frame extends along the side<br />Question and answers Electrical Maintenance Unit<br />- 272 -<br />of the coil, with its end forming a knife-edge on which the armature is pivotally<br />mounted. The armature is 'L' shaped and pivoted at its corner so that one arm can be<br />attracted to the end of the core while the other arm to operate a set of contacts.<br />Question and answers Electrical Maintenance Unit<br />- 273 -<br />• Purpose of Local breaker back-up relay; type -CTIG<br />To safe guard against the drastic consequences of failure to clear faults rapidly, many<br />power supply authorities install 2 independent systems of protection for major<br />transmission lines. There remains however the possibility of the circuit breaker itself<br />failing to operate and this hazard is traditionally covered by remote breaker back-up.;<br />CTIG relay is a 3 phase or 2 phase and earth fault instantaneous over current unit<br />intended for use with a time delay to give back-up protection in the event of a circuit<br />breaker failure. A particular feature of the CTIG relay is a fast reset, which enables<br />the time delay to be set closer to the breaker trip-time.<br />• Purpose of Battery earth fault relay; type- CAEM-21<br />The battery earth fault relay is used to detect earth faults and deterioration of wiring<br />insulation in either pole of battery. The scheme consists of a centre tapped resistor, a<br />measuring relay, plug setting bridge, auxiliary relay and rectifier bridge to provide<br />unidirectional supply to the measuring relay coil. For different battery voltages<br />different values of centre tapped resistors are used. Variable sensitivities are<br />provided by means of the tapped coil whose taps are connected to the plug setting<br />bridge. The centre tap of resistor is brought to one terminals of the relay and this<br />terminal is either directly earthed or earthed through a centre zero milli<br />ammeter. Under healthy condition no current flows through the measuring relay coil<br />and in any pole of the battery or wiring insulation failure, current flows through the<br />measuring relay coil and the relay operates.<br />• Purpose of Rotor earth fault relay (type- CAEM-33)<br />When a single E/F is detected in the DC field circuit of a machine, the machine has<br />to be taken out of service at the first opportunity. This is because, if allowed to run<br />with an E/F on the rotor, a subsequent second E/F can cause severe damage to the<br />machine. However, a relay like CAEM-33 which can detect such a second E/F and<br />trip out the machine can make it possible to run the machine even with a single E/F,<br />without any such risks, thus helping to preserve the generation capacity. The start of<br />the second rotor earth fault detection scheme is a very sensitivity transductor<br />element. The AC winding of the transductor is connected in series with a rectified<br />AC voltage relay. The Dc winding of the transductor on the other hand is connected<br />in series with the rotor E/F circuit. Under normal conditions- i.e. with no DC<br />flowing, the AC wining of the transductor presents a high impedance, and the AC<br />voltage applied is mostly dropped across this winding. Hence the relay remains deenergised.<br />When a second rotor E/F occurs, a DC current flows through the<br />transductor dc winding which causes the impedance of the AC winding to reduce<br />considerably by driving the transductor core into saturation. Hence, the applied<br />voltage is fully available across the relay and it operates.<br />Question and answers Electrical Maintenance Unit<br />- 274 -<br />• Purpose of Sensitive earth fault relay (type - CTUM-15 &amp; CTIGM-15<br />It may not be always possible to detect high resistance faults by convectional earth<br />fault relaying. In such cases a very sensitive current relay will be required for this<br />purpose. It can be connected residually since it has an adjustable definite time delay<br />provided to take care of transient spills in the residual circuit due to CT mismatch.<br />Also, its low burden enables it to be used with existing CT's/ Relays without<br />affecting the performance.; The incoming current is stepped down by an internal<br />current transformer and converted to a voltage by a variable resistor network. The<br />signal is compared with an internal reference. When this reference level is exceeded,<br />a time delay is initiated, after the time delay has elapsed, a relay operates.<br />• Purpose of Negative phase sequence current relay; Type- CTN/CTNM<br />Negative phase sequence current in the stator of a generator, caused due to<br />unbalanced loads or faults, it induces double frequency eddy current in the rotor.<br />These currents, if allowed to persist, can cause serious overheating and the purpose<br />of this relay is to disconnect the machine before such excess temperature is reached.<br />The inputs from the current transformers, which are connected in each phase of the<br />generator supply, are fed to a negative sequence filter which gives an AC output<br />voltage proportional to the negative sequence current. This voltage is rectified and<br />smoothed and fed into the squaring circuit of the main measuring element, the<br />definite time delay circuit and the alarm element. The output from the squaring<br />circuit is proportional to the square of the input voltage and is applied directly to the<br />main timing circuit to give the required relationship between I2<br />2t and the relay<br />operates time t.<br />• Purpose of definite time Over current &amp; earth fault relay: Type-CTU<br />This relay can be used for definite time over current protection against phase and<br />Earth faults on medium and low voltage distribution systems. The definite time relay<br />offers a considerable advantage over inverse time relays in instances where there ia a<br />wide variation in line impedance. Another application is in the field of stalling<br />protection of motors. When the thermal overload relay does not provide protection<br />against stalling, separate definite time O/C relay type CTU can be used to provide<br />the same. This relay comes in following nomenclature: CTU-12/22/32/52/62/15.<br />CTU relay combines the advantage of complete static measurements with<br />characteristic of the robust, well proved attracted armature unit. These relays<br />measure current and time accurately, imposes low burden on CT's. Each phase<br />comprises a static overload detector and timer, which is accurate over a 10:1 time<br />setting range. When the positive peak of the input signal exceeds the reference level,<br />the time delay circuit starts and after a preset time, drives the output relay.<br />Instantaneous high set unit when fitted uses alternate half cycle for measurement and<br />through a separate level detector drives a separate output relay.<br />Question and answers Electrical Maintenance Unit<br />- 275 -<br />• Purpose of Motor protection relay: Type- CTMM/CTMFM<br />This relay contains all the protection factors to protect the motor, from Thermal<br />overload (Ith), Instantaneous over current (I1), Instantaneous or time delayed<br />unbalance element, Earth fault Element (I0) &amp; Stalling protection (I1(t))<br />Question and answers Electrical Maintenance Unit<br />- 276 -<br />• Purpose of Overfluxing Relay: Type-GTTM<br />Transformers need protection against the risk of damage, which may be caused when<br />the transformers are operated at flux density levels significantly greater than the<br />design values. The overfluxing withstand time is generally found to be varying<br />inversely with the working flux density in the core, having higher withstand times<br />during extreme overfluxing conditions.<br />The overfluxing condition can occur during system over voltage or under frequency<br />conditions.<br />The basic operating principle is to produce an ac voltage, which depends upon the<br />ratio between AC input voltage and the frequency. The AC input voltage is fed to a<br />step-down transformer, which also provides isolation and the stepped down voltage<br />is fed to a V/F ratio detector circuit. This circuit is a simple operational amplifier<br />integrator with the provision for V/F pickup adjustment. The AC voltage is rectified<br />by true RMS. to dc converter. This circuit gives a frequency output and this<br />frequency increases rapidly with the increase in voltage. The frequency output is<br />given to a curve shaping circuit, which involves counter and comparators. The<br />counter counts the frequency output and the number of counts required for final trip<br />condition is changed by the comparator circuits to get the required timing<br />characteristic.<br />• Purpose of Biased Differential Relay: Type-MBCH<br />This relay is suitable for protection of two or three winding power transformers, auto<br />transformers or generators transformer units.<br />The differential transformer protection measuring circuit is based on the well-known<br />Merz-price circulating principle.<br />• Purpose of Digital frequency relay: Type-MFVUM<br />This relay is used to monitor the frequency of an electrical system. The relay are<br />suitable for any application in industrial plants and to generators where definite time<br />under or over frequency protection is required.<br />The operating principle of the relay is the comparison of the time interval of the<br />incoming frequency with that of a preset time derived from an accurate oscillator<br />within the relay. The incoming frequency signal is connected to an INPUT<br />CIRCUIT, which then drives an IMPULSE GENERATOR to produce a pulse at the<br />beginning of each period of the input voltage. The preset time interval is obtained<br />from an OSCILLATOR and COUNTER and adjustment is achieved using<br />SELECTOR switches, which drive a DECODER circuit. A COMPARATOR<br />compares the two-time interval and this triggers an adjustable TIMER which then<br />operates the output relay and latched light emitting diode (LED) glows.<br />• Purpose of Stator Earthfault Relay: Type-PVMM<br />Question and answers Electrical Maintenance Unit<br />- 277 -<br />A 100% stator earthfault protection is designed to detect earthfault occurring in the<br />regions of machine winding close to the neutral end. This relay is a composite<br />modular relay that gives 100% stator earthfault protection for machines, whose<br />neutral are not directly earthed. It works on the principle involving monitoring of the<br />neutral side and line-side components of the third harmonic voltages produced by<br />AC generators in service.<br />Question and answers Electrical Maintenance Unit<br />- 278 -<br />• Purpose of Voltage regulating relay and line drop compensator: Type-VTJCM &amp; CIJC.<br />This relay is used with on load transformer tap changers and induction regulators to<br />provide close and accurate automatic voltage regulation on power systems of any<br />voltage.<br />When the regulated voltage moves outside a dead band, set by the sensitivity control,<br />the volts high or volts low circuits are initiated and after a time delay, determined by<br />the response characteristic, the appropriate tap changer control auxiliary relay closes<br />its contact to initiate a tap change.<br />• Purpose of Directional power relay: Type-MWTU.<br />This relay setting ranges from 0.25% to 18.56% of rated power. This makes the relay<br />suitable for sensitive reverse power applications. For example with turbo-generator,<br />where the detection of 1% or 2% reverse power is necessary to prevent the<br />synchronous machine from motoring in the event of the power from the prime mover<br />becoming too low. It is also suitable for low forward power interlock and under<br />power protection.<br />• Purpose of Check synchronising relay: Type-SKD/SKE.<br />This relay is used to prevent interconnection of badly synchronised supplies. Type<br />SKD relay are used for auto reclosing sequence, type SKE relay are used to<br />safeguard manual synchronising of generators. Phase measurement is achieved by<br />algebraically subtracting the 2 supply voltage waveforms and comparing the<br />resultant modulated beat waveform envelope with a Dc reference voltage. The DC<br />reference is proportional to the sum of the peaks of the 2 supply voltages to provide<br />phase measurement independent of supply voltage variation.<br />• Purpose of Static distance protection: Type-SHPM.<br />This relay (QUADRAMHO) is a static distance protection relay specially designed<br />for comprehensive high-speed protection of HV &amp; EHV distribution/transmission<br />lines. 3 zones of protection are included, each employing separate measuring<br />elements, one element each for 3 phase-to-phase and 3 phase-to-earth faults. Thus a<br />total; of 18 elements are provided thereby increasing the reliability of the protection.<br />Poly phase measuring elements are not used in QUADRAMHO as in some of the<br />contemporary schemes. The relay is suitable for both three poles &amp; single-and-threepole<br />tripping of the circuit breaker.<br />• Purpose of Static offset MHO relay: Type- YTGM.<br />This relay is a static single phase, single step, and distance protection with MHO<br />offset MHO characteristic. With suitable current/voltage input selection, the relay<br />can be made to have the required characteristic in the R-X plane for various<br />applications such as Generator Field failure protection, Generator backup impedance<br />Question and answers Electrical Maintenance Unit<br />- 279 -<br />protection and as offset MHO relay for use in conjunction with generator pole<br />slipping protection.<br />Question and answers Electrical Maintenance Unit<br />- 280 -<br />• Purpose of sensitive power relay: Type-WCD.<br />This power relay is a sensitive Poly phase induction cup unit, providing under power,<br />reverse power and over power protection. This relay detects a reversal of current<br />flow, caused by insufficient driving power from the prime mover, preventing the<br />generator operating as a synchronous motor. The electrical quantities energize<br />windings on an eight pole laminated stator. The moving contact is operated by a cup<br />shaped Aluminium rotor, which turns on jewelled bearings in an air gap between the<br />stator and a fixed center core. Only a small arc of rotation is needed to cause contact<br />closer. Low rotor inertia and very high driving torque ensures a high speed<br />operations.<br />• Purpose of pole slipping relay: Type-ZTO.<br />This pole slipping relay has been designed to protect synchronous Generators against<br />the possibility of the machine running in the unstable region of the power angle<br />curve which would result in power oscillations and pole slip. The relay consists<br />basically of one directional relay and one blinder relay operating in conjunction with<br />a 40-80 milli seconds static timer. Intended primarily for installation between the<br />generator and associated transformer (preferably on the generator terminals)<br />• Purpose of fuse failure relay: Type - VAPM<br />This relay detects a failure or inadvertent removal of voltage transformer secondary<br />fuses and prevention of incorrect tripping of circuit breakers. This relay consists of a<br />rectified AC voltage operated hinged armature unit. Three coils for the three phases<br />are wound over a single core producing in effect a common relay for the three<br />phases. Each coil is connected across one of the voltage transformer secondary fuses<br />and under healthy conditions, this coil is short circuited by the fuse and cannot be<br />energized. When one or more fuses or are removed the appropriate coil(s) is<br />energized under relay operates immediately to open the trip circuit.<br />Question and answers Electrical Maintenance Unit<br />- 281 -<br />GENERATOR PROTECTION<br />Sl Des Description CT ratio Type Setting Remarks<br />1 87G Generator Differential<br />Protection<br />10000/5 CAG-34 P/S = 10% or 0.5 A SR = 143 Ohm<br />2 87GI Generator Inter Turn fault 5000/5 CAG-34 P/S = 10% or 0.5 A SR = 86.72<br />Ohm<br />3 32A Low Forward Power<br />Protection<br />10000/5 WCD-13 0.5% Fixed<br />2/32A Timer VTT-11 2.5 Sec<br />4 32B Low Forward Power Protection 10000/5 WCD-13 0.5% Fixed<br />2/32B Timer VTT-11 2.5 ec<br />5 32C Under Power Over speed limit 10000/5 WCD-12 30% Fixed<br />27 A,B Under Voltage Relay VAGM-22 73.2 V<br />6 37 Reverse Power Protection 10000/5 WCD-11 0.5% Fixed<br />2A/37 Timer VTT-11 10 Sec<br />2B/37 Timer VTT-11 5 Sec<br />7 21G1 Generator Back-up<br />Impdence Stage-1<br />10000/5 YTGM-15 K1=7.0, K2=1.0, K3=2.0,<br />K4=1.0, K5=1.0, K10= 0<br />Zf=14.0 Ohm, Zr= NA<br />2/21G Timer VTT-11 1.5 Sec<br />8 21G2 Generator Back-up Impdence<br />Stage-2<br />10000/5 YTGM-15 K1=1.65, K2=1.0, K3=2.0,<br />K4=1.0, K5=5, K10= -1<br />Zf=3.3 Ohm, Zr=10 Ohm<br />2/21G2 Timer VTT-11 2.0 Sec<br />9 40G Generator field failure 10000/5 YTGM-15 K1=6.175, K2=1.0, K3=4.0,<br />K4=1.0, K5=1.06, K10=+1<br />Zf=24.7.0 Ohm, Zr= 4.24<br />2A/40G Timer (TDDO) VTT-11 2.5 Sec<br />2B/40G Timer VTT-11 2.0 Sec<br />27/40G Under Voltage Relay VAGM-22 73.2 V<br />10 59G Over voltage Protection VTU - 21 Setting=120% + 2.0 Sec<br />11 78GY YTGM With pole slipping relay YTGM-15 K1=4.45, K2=1.0, K3=1.0,<br />10000/5 K4=1.0, K5=5, K10= -1<br />Zf=4.45 Ohm, Zr=5.0 Ohm<br />12 78G Pole Slipping Protection ZTO K1=0.98, K2=0.67, K3=4.0<br />10000/5 Q1=Q2=75 degree,<br />Timmer =54mSec<br />Over current Starter CAG-19 Current Setting=5.5A<br />13 64A 100% Stator E/F Protection PVMM-163 Vs=5.0 V, N=3<br />Neutral Displacement Module<br />Third harmonic Module VRL=70% Time=1.0 Sec<br />Third harmonic comporator unit This is to be set during<br />commissioning by Alstom engineer<br />14 64B 95% E/F Protection VDG-14 PSM=5.4 V, TMS=0.3<br />15 64C Stator E/F during Starting 300/1 CTIGM-15 Setting = 0.1 A<br />16 46G Gen. Negative phase sequence 10000/5 CTNM-31 I2S=5%, K1=6.7, K3= 1<br />Alarm=70%<br />2/46G Timer VTT-11 120.0 Sec<br />17 50 ABC Instantenuous Over current 10000/5 CAG-39 P/S = 5 A<br />18 49 G Generator Over load protn. 10000/5 CTMM - 104 Ith=4.4A, Thermal Ref. Curve=2a<br />19 50 S<br />ABC<br />Stator O/C Protn during starting 10000/5 CTIGM-15 Setting = 0.05 A<br />20 64 F1 First rotor E/F protection VAEM - 21 Setting = 1.1 mA fixed<br />21 2/64F1 Timer VTT-11 Setting = 2.0 Sec<br />Question and answers Electrical Maintenance Unit<br />- 282 -<br />22 64F2 Second rotor E/F Protection CAEM-33 Setting = 1.0 mA fixed<br />23 81 - 1 81-1RL1 Under Frequency Stage - 1 MFVUM Setting = 47.96 Hz + 1.1 Sec Window annun<br />81-1RL2 Under Frequency Stage - 2 Setting = U#1=47.6 Hz<br />+2Sec,U#2=47.7Hz+3 Sec<br />SUT breaker trip<br />Setting = U#1=47.6 Hz +4<br />Sec,U#2=47.7Hz+ 4sec Sec<br />86C Operation<br />24 81 - 2 81-2RL1 Under Frequency Stage - 3 MFVUM Setting = 47.5Hz +0.1Sec SUT breaker trip<br />Setting = 47.5Hz +0.6 Sec 86C Operation<br />25 81 - 3 81-3RL1 Over Frequency Stage - 1 MFVUM Setting = 51.0 Hz + 1.1 Sec Window annun<br />81-3RL2 Over Frequency Stage - 2 Setting = U#1=51.5 Hz + 15<br />Sec,U#2=51.65 Hz + 15 Sec<br />86BG Operation<br />26 81 – 4 Rate of rise of frequency (<br />df/dt)<br />MICOM Setting = 50.01+2.0Hz/Sec + 0.2Sec SUT breaker trip<br />Setting = 50.01+2.0Hz/Sec + 0.5Sec 86C Operation<br />Over Frequency ( f+t) Setting = 51.5 Hz + 0.2 Sec SUT breaker trip<br />Setting = 51.5 Hz + 0.5 Sec 86C Operation<br />GENERATOR TRANSFORMER PROTECTION<br />Sl Designation Description Type Setting<br />1 87GT Over all Differential<br />Protection<br />MBCH-13 Settings = 20%<br />2 50/51GT Back-up O/C HV side CDG-63 PSM=1.0, TMS=0.4, Inst = 800%<br />3 50Z GT Breaker L.B.B. Protn. CTIG-39 Setting = 5%<br />2/50Z Timer VTT-11 Setting = 250 mSec<br />4 64GT G.T. H.V. REF CAG-14 Setting = 0.1 A, SR=185 Ohms<br />5 51 NGT G.T. B/U E/F Protn. CDG-11 PSM=0.2, TMS=0.52<br />2/51NGT Timer VTT-11 Setting = 1.0 Sec<br />6 30 FG WTI Alarm WTI Set 90 degree C alarm<br />7 30EG WTI Trip WTI Set 100 degree C trip<br />8 30 HG OTI Alarm OTI Set 70 degree C Alarm<br />9 30GG OTI Trip OTI Set 80 degree C Alarm<br />10 99G1 GT Over flux Stage-1 GTTM-22 Setting K1=1.1, K2=1.3<br />2/G1A Timer VTT-11 Setting = 10.0 Sec<br />11 99G2 GT Over flux Stage-2 GTT-21 Setting V/F=1.15, 99G2A=1.0Sec<br />99G2T=120 Sec<br />2/G2A Timer VTT-11 Setting=10.0 Sec<br />UNIT AUXILIARY TRANSFORMER PROTECTION<br />Sl.<br />No<br />.<br />Designation Description Relay Type Settings<br />1 87 UAT UAT Differential Protection MBCH-13 Setting=20%<br />2 64UAT A /<br />64UAT B<br />UAT REF Protection FAC-14 Setting=125 V<br />3 50/51 UT UAT B/U O/C Protection CDG-63 PSM=1.0, TMS= 0.32, Inst= 600%<br />4 51SN1/51SN2 B/U E/F UAT LV-A CDG-11 PSM=0.2, TMS= 0.44.<br />6 WTI Set 88 degree C alarm<br />7 WTI Trip WTI Set 93 degree C Trip<br />8 OTI Alarm OTI Set 80 degree C Alarm<br />9 OTI Trip OTI Set 90 degree C Trip<br />10 AVR Automatic Voltage Regulator VTJCM-13 1. Regulated voltage=110 V<br />2. Sensitivity: Dead band=+/- 2.5%<br />3. Selected Characteristics "c"<br />11 50 RYB OLTC O/C Protection CAG-39 95% I.e., 0.95A<br />Question and answers Electrical Maintenance Unit<br />- 283 -<br />START-UP TRANSFORMER PROTECTION<br />Sl.<br />No.<br />Designation Description Relay Type Settings<br />1 87 SUT ABC Differential Protection MBCH-13 Setting = 20%<br />2 64 HV REF Protn. HV Side FAC-14 Setting = 25V<br />3 64LVA/LVB REF Protn. LV Sides FAC-14 Setting = 125 V<br />4 67 ABC Directional O/C Protection CDD-41 PSM=0.75, TMS=0.4, Inst=600%<br />5 67 N Directional E/F Protection CDD-41 PSM=0.2, TMS=0.4, Inst=200%<br />6 99SUT Over fluxing Protection GTTM-22 Settings K1=1.1, K2=1.23<br />7 51SNA/51SNB LV Side B/U E/F Protn. CDG-11 PSM=0.2, TMS=0.4.<br />8 50Z Local Breaker B/U protn. CTIG-39 Setting=0.2A<br />2/50Z Timer VTT-11 Setting = 0.25 Sec<br />9 WTI Winging Temperature 95 degree C Alarm<br />105 degree C Trip<br />10 OTI Oil Temperature 85 degree C Alarm<br />95 degree C Trip<br />11 AVR Automatic Voltage<br />Regulator<br />EMCO EE-<br />301-M<br />1. Regulated Voltage = 110 V<br />2. Nominal Value = 110 V<br />3. L Setting = 2.75V (2.5%)<br />4. R Setting = 2.75 V (2.5%)<br />5. Time delay setting = 20 Sec<br />12 81-5 SUT Over Frequency Protn. MFVUM-22 52.0 Hz + 20.0 Sec<br />Question and answers Electrical Maintenance Unit<br />- 284 -<br />CT’s, PT’s and PROTECTION<br />• What is the inrush current peak of the transformer?<br />6 to 8 time that of full load current.<br />• Why REF is now is used for HV side also in GT/SUT?<br />The E/F setting of differential is poor.<br />• Why IDMT over current relay is always used as backup?<br />Because setting has to be 200% to emergency loading and TMS be large to grade<br />with feeder. Therefore very slow for internal faults/terminal faults/uncleared LV<br />faults.<br />• Purpose of standby E/F protection in SUT/UT?<br />Back up for LV winding, LV neutral CT- CDG 12 – resistance earthing – relay set<br />high time delay to discriminate with LV feeder and trip transformer if sustained E/F,<br />also protects neutral earthing resistor.<br />• Why do we use O/C &amp; E/F protection on both sides of transformer?<br />Power in feed exists on both ends.<br />• Why in DG E/F protection, we do not open class IV CB’s or supply CB’s?<br />Delta of aux. Transformer prevents E/F currents from grid into DG neutral.<br />• Why 100% winding protection is felt essential for main generator stator E/F<br />protection? (Used in NAPS onwards?)<br />At MAPS 4% of winding is not protected. Earlier felt that the Electro magnetic stress<br />due high external fault currents near 4% of neutral may not be high to cause E/F<br />here. But now felt that the mechanical stress can leads to E/F.<br />• How 100% winding protection is given there?<br />a) Inject sub harmonic AC current into generator neutral. Monitor its amplitude. E/F<br />impedance reduces so current drawn increases and trips (Not used).<br />b) 3rd harmonic voltage monitored on neutral, fault near neutral upto 25% winding.<br />3rd harmonic voltage reduces to zero. Above this 3rd harmonic voltage increases,<br />so combined both 3rd harmonic and zero sequence relays for 100% covering, no<br />blind zone.<br />• What is the basic purpose of class-B protection?<br />Class-B avoids load rejection. For modern machines, the inertia is less and easily<br />gets damaged on overload. Therefore trip only for internal faults.<br />Low forward interlock prevents the risk of run away if a CIES valve fails to close.<br />Question and answers Electrical Maintenance Unit<br />- 285 -<br />Question and answers Electrical Maintenance Unit<br />- 286 -<br />• What are the effects of GT over fluxing?<br />a. Eddy current circulation.<br />b. Magnetising current increases<br />c. Winding temp increases<br />d. Transformer noise/vibration increases.<br />e. Over heating of non laminated metal parts (affected by stray fluxes)<br />• Why stabilising resistor in REF or residual E/F scheme?<br />Required against CT saturation under heavy through fault currents.<br />• Why in transformer the LV CB also be tripped along with HV CB for a primary side<br />fault?<br />Auxiliary transformer 415v delta star transformer, if HV CB alone tripped then back<br />feeding from LV side (say DG runs parallel with transformer)—arcing voltage at the<br />fault on primary—fault fed for more time – more damage.<br />• Why high impedance circulating current differential?<br />Under through faults, CT’s of different phases saturates differently. Net spill current<br />will operate low impedance CAG relay, so high impedance scheme with CAG<br />relay and stabilising resistor used.<br />• How to reduce the CT error?<br />Error reduces if load increases.<br />• What is the advantage of housing CT’s with in bushings?<br />Bushing acts as a primary insulator for the CT.<br />• Why the earthing transformer primary voltage is 16.5 kV rated in main generator<br />even though actual voltage during the E/F is root 3 times less?<br />The transformer should not saturate during E/F otherwise it will cause<br />ferroresonance with the GT winding capacitance. Dangerous O/V and neutral<br />shifting will occur. During loss of load or field forcing conditions, the transformer<br />voltage increases to cause saturation. Saturation can also occur due to point on wave<br />of application causing flux doubling.<br />• Where are the following relays used?<br />a) Very inverse b) extremely inverse relays c) definite time O/C Relay d)<br />instantaneous O/C Relay.<br />a) Very Inverse – Used where inverse protection reduces substantially as distance from<br />source increases, operating time doubles for a fault current reduction from 7 in to 4 in,<br />used where the short ckt current is independent of generating conditions.<br />Question and answers Electrical Maintenance Unit<br />- 287 -<br />b) Extremely inverse – Used for feeders subjected to peak making currents. Grade with<br />HRC fuses, e.g. Refrigerator, pumps.<br />c) Definite time O/C Relay – Where neutral is resistance earthed- fixed ground current.<br />d) Instantaneous O/C Relay – Used along with inverse O/C relay – to get higher grading<br />margin. Disadvantage – Under minimum generation it may not operate.<br />Question and answers Electrical Maintenance Unit<br />- 288 -<br />• Why delta – delta CT’s are used for star – star transformer differential protection?<br />Say primary neutral is not solidly earthed. Then for any earthfault on secondary<br />terminal, the primary current distribution is so for external fault, the differential is<br />likely to operate if sequence current from flowing into relay. The 2:1:1 distribution is<br />possibly only for core type or delta tertiary.<br />• Show the CT characteristics.<br />Knee point region (Protection characteristics)<br />Saturation region<br />Peak flux density<br />Linear region<br />Ankle point<br />(Measuring CT characteristics)<br />RMS amp turns<br />• What is knee point?<br />Knee point is the region, where 10% increase in flux causes 50% increase in exciting<br />ampere-turns.<br />• When will you say that the CT is saturated?<br />When checking the CT with the secondary injection method a 10% increases in the<br />voltage causes a 50% increase in the current the CT is said to be saturated.<br />• What is the problem anticipated due to CT saturation?<br />The CT will not be able to drive the current through the circuit causing nonoperation<br />of relays. In some other case when the currents in the two phases are<br />compared for relay operation the relay may operate due to unbalance.<br />• How can you de-saturate the CT?<br />Pass ac current through the primary and vary the current from zero to maximum with<br />secondary in shorted condition.<br />Pass dc current in the secondary and vary it from zero to maximum.<br />• Why CT should not be open circuited?<br />Very high voltage will be induced in secondary due to less back emf resulting in the<br />failure of the insulation.<br />• What precaution should be taken while removing a current operated relay when the<br />equipment is in service?<br />Question and answers Electrical Maintenance Unit<br />- 289 -<br />Ensure that the CT is not getting opened by shorting the appropriate terminals.<br />(Eventhough the terminals are automatically shorted once relay is removed the above<br />point may carried out to ensure the same)<br />Question and answers Electrical Maintenance Unit<br />- 290 -<br />• What do 10p15 mean?<br />When the current passed through the CT is 15 times the rated current then the<br />secondary current will have a composite error of 10%<br />• Where core balance CT’s are used?<br />In earth fault protection used. It senses the zero sequence current.<br />• What are the specifications of CT?<br />Protection CT - Error. Alf. KpV.<br />Metering CT - Error. Burden.<br />Differential CT - Class PS.<br />Core balance or E/F CT - 5-p type.<br />Primary current -<br />Rating of CT - 1. 15 ( full load current )<br />Short time rating - 1 sec.<br />• Why differential protection for PHT motors?<br />For more than 2500 kW motors it is required to provide differential protection. It is<br />biased Relay against internal phase fault or earth fault very fast. Insensitive to<br />starting current and stalling current.<br />• What are the errors of the following CT’ s 5p. 10p. 15p. At rated current?<br />5p - 1 % Ratio error ± 60 min phase error<br />10p - 3 % Ratio error ± 60 min phase error<br />15p - 5 % Ratio error ± 60 min phase error<br />• What is the operating point in the Magnetising characteristic of protection CT &amp;<br />measuring CT?<br />Protection CT – Operation at ankle point only.<br />Measuring CT – Operation from ankle to knee point<br />• What is over voltage interturn test for CT?<br />With secondary open, pass rated current in primary for 1 min. Then check secondary<br />for insulation.<br />• A CT has 2 – secondary windings. If we use only one secondary winding can we<br />keep the unused secondary winding short circuited?<br />No. If it is short-circuited then the ratio will not get correctly. The turns of primary<br />winding will be shared between 2 secondary windings. So the unused secondary<br />winding should kept open.<br />• But is it advisable to keep the secondary of CT in open conditions? Will not induce<br />very high voltage?<br />Question and answers Electrical Maintenance Unit<br />- 291 -<br />If the CT has only one secondary winding, we should keep it always short cktd for<br />safety, but if the CT has multiple secondary, then if one secondary voltage is kept<br />limited by suitable loading, then the other secondary voltage is eventually limited<br />proportionately.<br />Question and answers Electrical Maintenance Unit<br />- 292 -<br />• Why PT fuse fails protection?<br />Mho relays will mal-operate if PT voltage is lost to the relay, so tripping blocked by<br />sensing PT fuse failure.<br />• What is the 2 stage stalling protection for PHT motor?<br />Because locked rotors withstand time of motor is less than starting time of motor<br />under reduced voltage conditions.<br />Stage 1 = 350% 6 sec for starting at rated voltage<br />(Because starting time = 6 sec + hot stall time = 7 sec)<br />Stage 2 = 175% 15 sec to permit 14 seconds starting time under reduced voltage<br />condition<br />(Since starting current is less, stage 1 will not operate)<br />• Purpose of start up protection? Is it always in service?<br />Trips the generator. If generator is excited with internal fault the over current 50s trip<br />the generator to prevent major damage. The earth fault relay 64c also. The relays are<br />polarised dc armature type, sensitive to all frequencies, since the frequency need not<br />to be 50 Hz initially during start up. Start up protection is cutout as soon as generator<br />CB is closed.<br />• What is the standard CT polarity?<br />Primary current enters at P1 and secondary current leaves at S2.<br />• Does over load relay give 100% guarantee against the single phasing?<br />No. It depends on the motor load and the motor winding (star or delta).<br />• What are the effects of single phasing?<br />a. Current will increase √3 times.<br />b. More heat in stator and rotor parts.<br />c. Insulation failure and short circuit &amp; Ground fault may occur.<br />• What is the purpose of CT &amp; PT?<br />For transformation of current, voltage to a lower level for the purpose of<br />Measurement, Protection and Control.<br />• Where CT secondary of 1A we are using?<br />For long distance current transmission, to reduce the IR drop.<br />• What is the nomenclature of English electric relay?<br />a) First letter-operating quantity<br />b) Second letter-movement<br />c) Third letter-application<br />Question and answers Electrical Maintenance Unit<br />- 293 -<br />d) Fourth letter-special variation.<br />• Define knee point voltage.<br />The voltage applied to secondary of CT keeping the primary open at which<br />10% increase in voltage causes 50% increase in excitation current.<br />Question and answers Electrical Maintenance Unit<br />- 294 -<br />• What is differential protection?<br />It is the current balance type protection, in which vector difference between current<br />entering the winding is used for relay operation.<br />• What are the checks on CT &amp; PT?<br />a) Polarity checks<br />b) Insulation checks<br />c) Ratio checks<br />d) Knee point voltage (only for PS class CT)- magnetising characteristic test.<br />• What is Local Breaker Back up protection?<br />In case of local breaker fails to operate during fault due to mechanical failure this<br />protection will protect the system from sever damage. It will trip all the other<br />breakers in that bus after time delay.<br />Question and answers Electrical Maintenance Unit<br />- 295 -<br />TRANSFORMERS<br />• Give transformer nameplate details of GT, SUT, UAT, SET, 415 V Aux transformer<br />and Lighting transformer.<br />GT SUT UAT SET 415V Aux trans. L Trans.<br />USI 5210 5120 5220 4120 5242 5231<br />Make Telk Telk BHEL BHEL EE Square<br />Automation<br />STD IS – 2026 IS– 2026<br />Type WFOC Oil immersed Oil immersed DRY RESIN<br />CAST DRY RESIN CAST DRY RESIN CAST<br />Cooling OFWF ONAF / ONAN ONAF / ONAN AN AN AN<br />VA 260/260 MVA 35/20/20/12 MVA<br />24.5/14/14 MVA<br />35/20/20 MVA<br />24.5/14/14 MVA<br />3150 kVA 1800/1200 kVA 250 kVA<br />Volts 235/16.5 kV 220/6.9/6.9/11<br />kV<br />16.5/6.9/6.9<br />Kv<br />16500/575<br />V 6600/435 V 415/415V<br />Amps 639/9098 A 64/1172/440 A<br />91/1675/629 A<br />1266/858A<br />1676/1172 A 157.5/2400 A 630/250,125A<br />No of φ 3 3 3 3 3 3<br />Frequency 50 Hz 50 Hz 50 Hz 50 Hz 50 Hz 50 Hz<br />Impedance 13.13 (14) % 9.75% / 18.82% 10 ±10% HV<br />22 ±10% LV<br />Vector YNd11 Yn yno yno<br />(d1)<br />D yn1 yn1 Dyn Dyn11 Dyn11<br />Oil 42000 Lt. 25260 Lt. 19750 Lt.<br />Tap change Off load ON load HV ON load HV<br />Tapchange% 10 steps of 2.5 % ϒ12% in 1.5% steps ϒ12% in 1.5% steps<br />• What is the use of Tertiary winding?<br />Star connected circuit, which has an isolated neutral there can be no zero sequence<br />components. Since the zero sequence components are by definition in time phase<br />with another their sum can not be zero at the junction point as per kirchoff’s law. It<br />follows that there are limitations upon the phase loading of a bank of transformers<br />connected in star – star unless the neutral points are connected to the source of power<br />in such a manner that the zero sequence components of current have a return path or<br />unless the transformer are provided with tertiary winding.<br />• What is E/F current limit for SUT and UT?<br />400a limited by 10 ohms resistor.<br />• What is the coverage of differential protection for SUT?<br />Covers from 230 kV bushing to 6.6 kV breaker end.<br />• What are the advantages of dry type transformer?<br />No fire hazard.<br />It can be mounted indoor.<br />Question and answers Electrical Maintenance Unit<br />- 296 -<br />• During unit operation, can we parallel 2 SUT?<br />No, due to switchgear limitation.<br />Question and answers Electrical Maintenance Unit<br />- 297 -<br />• Why 6.6 kV transformer is resistance grounded by 10 ohms and current limited to<br />400 A?<br />a) To reduce burning and melting in faulted switchgear or machine.<br />b) To reduce mechanical stresses in equipment.<br />c) To reduce the electrical hazards by stray ground fault currents in the ground<br />return path.<br />d) To reduce momentary line voltage dip due to ground fault.<br />e) The current is limited to 400a, that is ¼ th of the load current to reduce the size of<br />the screen in 6.6 kV XLPE (cross-linked polyethylene). Therefore the cost of the<br />cable decreases.<br />• During unit operation can we have one UT feeding both unit 6.6 kV loads?<br />No, logically prevented.<br />• During unit operation, can we parallel UT &amp; SUT continuously?<br />No, due to switchgear limitation.<br />• What is the design basis of 6.6 kV aluminium bus bars?<br />a) Temperature rises not exceed 90 ºC.<br />b) Withstand short ckt stresses.<br />c) Take care of thermal expansion.<br />• Why 2 types of earth fault relays in 6.6 kV side of transformers?<br />I – Trips 6.6 kV breakers only. It gives primary protection for 6.6 kV bus bars.<br />I1 – Trips the both HT and LT breakers. It acts as a backup to ref and also acts as<br />backup to bus bar earthfault relay.<br />• Why core balance CT is preferred over residual connected CT’s to sense earth fault<br />in 6.6 kV feeders?<br />a) To avoid relay mal-operation due to CT saturation<br />b) Better sensitivity is got.<br />c) High pickup and TMS avoided in IDMT earth fault relay.<br />• How selection of cooling fluid in GT done?<br />a) There are 5 factors are there.<br />b) Density<br />c) Coefficient of thermal expansion<br />d) Viscosity<br />e) Specific heat<br />f) Thermal conductivity.<br />• What are the ranges in which each type is effective?<br />Question and answers Electrical Maintenance Unit<br />- 298 -<br />ONAN – Natural cooling – up to 15 MVA.<br />ONAF – Air forced radiators cooling – 10 to 100 MVA depending on availability of<br />area.<br />OFWF = oil forced and water forced used in more than 100 MVA.<br />Question and answers Electrical Maintenance Unit<br />- 299 -<br />• Why off load tap changer was chosen for GT?<br />Because our plant works on base load always.<br />• What are the advantages of OFWF?<br />Ensure the differential temperature between top and bottom of transformer is<br />minimum and Effect of ambient air temperature is minimum.<br />• What is the type of lightning arrestor for GT?<br />Zn O (zinc oxide) types.<br />• What is the purpose of header breaker in water circuit?<br />The header breaker ensures oil pressure greater than that of water pressure always.<br />Therefore there is no leak of water into oil.<br />• Why thermosyphon filter required?<br />To keep required dryness/improve dryness of the transformer insulation, internal part<br />of transformer. When transformer operates, due to pressure head between top and<br />bottom small quantity of oil flows through filters (absorbent material activated<br />alumina grade g-80 removes the moisture from oil). Absorbent material remove<br />slag, acids, peroxides, ionic impurities from oil, which otherwise accelerate<br />against of oil. Absorbent unit is reactivated at regular intervals.<br />• What is the purpose of pronol conservator (KAPP)?<br />Flexible separator avoids direct contact with atmosphere. Efficient barrier between<br />oil and air. Ensures the protection against water vapour, suppression of gas bubbles<br />formation in the oil.<br />• Why main generator/UT is not provided with separate overfluxing protection?<br />Since GT is provided with overfluxing protection, it is adequate to protect main<br />generator / UT also. Main generator can withstand higher degree of overfluxing. If a<br />generator CB is used, separate overfluxing protection is essential for main generator.<br />• What is the advantage of Pressure relief device in TELK type GT over explosion<br />vent of BHEL, even though in both cases oil will be expelled out during sudden<br />pressure rise?<br />During internal fault, the internal pressure rise is relieved by the expelling out of oil<br />through Pressure relief device /explosion vent. However the Pressure relief device<br />closes back when the pressure drops. Hence the oil exposure to atmosphere is<br />minimised, thus saving large quantity of costly transformer oil from oxidation and<br />moisture absorption. Fire hazard due to transformer oil does not exist after the<br />closure of Pressure relief device.<br />Question and answers Electrical Maintenance Unit<br />- 300 -<br />• To reduce tower-footing resistance, which are better to use a) chemical, b) ground<br />rods, c) counter poise?<br />B &amp; C<br />Question and answers Electrical Maintenance Unit<br />- 301 -<br />• Why tap changer is kept at neutral end?<br />a) To reduce insulation cost of tap changer.<br />b) But reactance changeover the tap range increases.<br />• Why guard connection is given for megger?<br />For true measurement of IR value of HV to earth of a transformer, connect line to<br />HV, earth to transformer tank and guard to LV. Therefore leakage current from HV<br />to LV is not included.<br />• Why lighting isolation transformer is req.?<br />a) 3 wire to 4 wire conversion, since neutral is required for lighting load.<br />b) Prevents transfer of E/F currents<br />c) Reduces the fault level on secondary side and permits use of small sized cables /<br />CB’s / fuses.<br />• Why neutrals are solid grounded above 33 kV?<br />a) Less transient over voltage due to arcing grounds.<br />b) Voltage of phases are limited to phase to ground voltage. (No neutral shifting)<br />c) Allows graded insulation of transformer (low cost)<br />d) Fast E/F protection.<br />• Why SET is chosen as Dyn 11?<br />To have smooth commutation in generation in between stator and rotor.<br />• Why all 415V transformers are chosen Dyn 11? What are the protections provided<br />for the 415V transformers?<br />a) To facilitate interchange.<br />b) To have momentary parallel during changeover.<br />Protections<br />a) Door interlock to trip HT and LT breakers.<br />b) LT breaker can on only after HT breaker is in on position.<br />c) Instantaneous O/C and inverse O/C (50 + 51).<br />d) Instantaneous E/F (50N).<br />e) IDMT E/F and restricted E/F (51N + 64).<br />f) Winding temp high trip (140°C trip and 130°C alarm)<br />• What is the instrument name used for thermograph?<br />Infrared camera.<br />• Why neutrals are solid grounded below 600v?<br />Question and answers Electrical Maintenance Unit<br />- 302 -<br />Human safety<br />Permits enough E/F current because ground resistance is large in less than 415v,<br />hence fast fault clearance,<br />Equipment safety against over voltage.<br />Question and answers Electrical Maintenance Unit<br />- 303 -<br />• What are the advantages of ungrounded system?<br />Supply is maintained even with fault on one line<br />Less interference to communication lines because of absence of zero sequence<br />currents.<br />• Why resistance grounding preferred for less than 33 kV and more than 415 V?<br />a) To limit the earth fault current for equipment safety else, high short ckt forces<br />dislocate in windings/bus bars etc,<br />b) Over voltage due to arcing ground reduced<br />c) Permits earth fault protection (not possible in ungrounded system)<br />• What is meant by tan-delta measurement?<br />It is the tan of the angle between the capacitive current and the total current.<br />Ir<br />Ic Ic - capacitive current<br />I Ir - resistive current<br />I - total current<br />As the value of tan delta increases the resistive component of the current in<br />increasing. Hence it shows a weak insulation.<br />• What is the vector group of GT, UT, SUT?<br />Yd11<br />Dy1<br />Yy0<br />• Why all the transformers are having different vector group?<br />UT and SUT are getting paralleled at 6.6 kV bus. Hence they should have voltage of<br />same phase relationship. This is achieved by assigning different vector group to the<br />transformers.<br />• What are the built in protections for transformers?<br />a) Buchholz relay<br />b) Explosion vent or relief valve<br />c) Gas operated relay for on load tap changers.<br />• Why water pressure is kept below the oil pressure? How it is maintained?<br />Incase of a heat exchanger tube failure the water should not go inside the<br />transformer. For this purpose the oil pressure is kept above the water pressure.<br />Question and answers Electrical Maintenance Unit<br />- 304 -<br />• What is the requirement oil in a transformer?<br />Oil is used removal of heat produced in the transformer and also as insulating<br />medium.<br />Question and answers Electrical Maintenance Unit<br />- 305 -<br />• What is meant by over fluxing of transformer?<br />When the voltage is increased and the frequency is reduced the transformer will draw<br />high magnetising current. This will result in higher core loss and subsequent heating<br />of core and ultimate failure of transformer. Hence over fluxing protection is provided<br />for the transformer.<br />• What type oil pumps are used?<br />Canned rotor pumps.<br />• What is oil reclaiming and reconditioning?<br />In reclaiming process the oil treated to remove all its impurities like acidity, sludge,<br />sediments, moisture etc. The treated oil will be in par with the new oil. In<br />reconditioning process (filtering of oil) only moisture and suspended impurities and<br />sediments are removed.<br />• Why there is no mixing of oil of tap changer and transformer?<br />When the tap changing takes place arc is struck between the contacts. Due to this the<br />oil inside the tap changer will be highly carbonised. If both oil get mixed up the<br />quality of transformer tank oil will come down. This is not advisable. Hence both<br />oils are kept separately.<br />• Why the tap changers are always connected to HV side of the transformer?<br />During tap changing action the load current has to be shifted from one tap to another<br />tap. In case HV wining the load current will be less. Hence lesser arcing will take<br />place.<br />• What is the purpose of conservator?<br />To accommodate the change in volume of oil during increase in temperature.<br />• Why the neutral is earthed through earthing resistance in case of UT and SUT?<br />This is done to limit the earth fault current.<br />• Why REF is provided in the LV side of SUT and UT?<br />The LV sides of the two transformers are earthed through the resistance. This will<br />limit the flow of current in case of LV earth fault. Hence the differential protection<br />may not act for a LV earth fault. Hence ref protection is provided.<br />• Why twin secondary SUT?<br />As per is, the rating single secondary power transformer is limited to 25MVA (6.6<br />kV) or 40 MVA (11 kV), in order to limit the 3 phase symmetrical fault level with in<br />26-40 kA (contribution from grid and local machines)<br />Question and answers Electrical Maintenance Unit<br />- 306 -<br />• How 6.6 kV-bus supply was chosen?<br />11 kV was rejected in view of the high insulation cost with 11 kV motors.<br />3.3 kV was rejected, since max motor size with 3.3 kV bus is limited to 2 MW. But<br />we are having the motors having rating more than 2 MW, which cannot suit to 3.3<br />kV bus. 6.6 kV bus we can start upto 5 mw size motor.<br />Question and answers Electrical Maintenance Unit<br />- 307 -<br />• Why oil transformers are out door?<br />Oil fire point = 170 ºC easy catching of fire.<br />• What are the I.S used in transformers?<br />IS – 1866 FOR MAINTENANCE AND SUPERVISION OF OIL<br />IS – 10593 FOR GAS ANALYSIS<br />IS – 1886 FOR INSTALLATION AND MAINTENANCE<br />• When oil filteration is required?<br />On reweaving oil test results.<br />Draining of oil for maintenance<br />Topping up of transformer oil<br />• Why oil filteration is required?<br />To remove water, sediments, sludge etc.<br />• What are the types of oil used for in transformer for cooling?<br />Paraffin based and naphtha based (in INDIA)<br />• What are the types of bushing used in transformer?<br />Condenser type bushing<br />Porcelain type bushing<br />• What are the precautions to be taken while terminating the bushings?<br />Contact surfaces with intermediate plates,<br />Mating surfaces should be identical.<br />• How bushings are terminated inside the transformers?<br />By grooving method or by binding wire method.<br />• Why ICT are used?(INTERPOSING CT)<br />To correct the system primary CT errors in case of high current faults out side CT<br />zone (ICT’s primary CT is 800/1, but in fault current may go to thousands of amps.<br />This ICT will take care of those errors.<br />a) Matching the ratios.<br />b) Matching the phase angle differences.<br />• How CT is connecting in ckt?<br />If the primary of CT is delta connected load the CT will be in star connection and<br />vice versa. This is because to have square root 3 time compensation.<br />Question and answers Electrical Maintenance Unit<br />- 308 -<br />• What type of gasket and adhesive are used in transformer?<br />Gasket – Neoprene based rubberised cork type RC70-C. (IS4253)<br />Adhesive –Dunlop adhesive S-758<br />These are recommended by TELK<br />Question and answers Electrical Maintenance Unit<br />- 309 -<br />• What are the precautions to store the Gasket?<br />a) Stress free storage<br />b) No folding<br />c) No reuse<br />d) Replace with same thickness<br />• What is the in built protection for transformer?<br />PRV to protect from over pressurization of tank due to the release of gases, oil etc.<br />This is the replacement for the explosion vent.<br />• Why UT, SUT secondary is rated for 6.9kV where as bus voltage is 6.6kV?<br />The no load secondary 6.9kV voltage level adequately takes into account voltage<br />drop during loaded condition to cater station buses at 6.6kV level.<br />• Why our GT having off load tap changer?<br />Because our station is base load station.<br />• Why vector group of SUT is chosen as Yn-Yo-Yo?<br />To facilitate momentary paralleling of SUT with UT on 6.6kV buses.<br />• Grounding of various transformers.<br />GT HV solidly grounded<br />LV (delta)<br />UT HV (delta)<br />LV cast stainless steel 9.95 ohms 400A for 10 seconds.<br />SUT HV solidly grounded<br />LV cast stainless steel 9.95 ohms 400A for 10 seconds.<br />• What are the protections for GENERATOR TRANSFORMER?<br />a) Differential protection<br />b) Restricted earthfault protection<br />c) Backup earthfault protection<br />d) GT phase back up protection<br />e) Overfluxing protection<br />f) Oil surge (gas) protection<br />g) High winding temperature and oil temperature protection.<br />• What are the protections for SUT?<br />a) Over current protection for phase and earth fault<br />b) Differential protection<br />c) HV and LV restricted earthfault protection<br />Question and answers Electrical Maintenance Unit<br />- 310 -<br />d) HV side directional back up over current protection for phase and earth fault.<br />e) LV back up over current and earth fault protection<br />f) Over fluxing protection<br />g) Buchholz and high oil, winding temperature protection.<br />Question and answers Electrical Maintenance Unit<br />- 311 -<br />• What are the protections for UT?<br />a) Differential protection<br />b) LV restricted earthfault protection<br />c) LV back up earthfault and over current protection<br />d) Buchholz and high oil, winding temperature protection.<br />• What is the purpose of carona ring?<br />To minimize the arcing current during switching operations of disconnecting<br />switches.<br />• What are the various tests on transformers?<br />a) Tan delta and capacitance dissipation factor<br />b) Tests on cooling fans<br />c) Tests on OLTC<br />d) Vector group test<br />e) Short circuit test<br />f) Open circuit test<br />g) Insulation resistance test<br />h) Turns ratio test<br />i) Winding resistance test.<br />• Why input transformer of PUPS module 1 is delta-delta and module 2 is delta-star?<br />With the help of this arrangement, combined DC output from both chargers is<br />equivalent to that from a 12-pulse rectifier. Advantage of 12-pulse rectifier is that the<br />mains current is fairly close to sine wave. Harmonics injected into system by rectifier<br />are low. The phase angle difference 30-degree between module 1 output and module<br />2 output give 12-pulse output.<br />Question and answers Electrical Maintenance Unit<br />- 312 -<br />Question and answers Electrical Maintenance Unit<br />- 313 -<br />MOTORS AND MCC<br />• What is Motor?<br />It is a device, which converts electrical energy into mechanical (rotating) energy.<br />Definition of terms used in Motor:<br />1) Duty Cycle rating: Most motor has a continuous duty rating to permit continuous<br />operation at a rated load. However motors may be rated as intermittent duty,<br />periodic duty or varying duty and must be turned off and allowed to cool after a<br />fixed operating time.<br />2) Full- Load current: The current required to produce full-load torque.<br />3) Jogging: The starting and stopping of a motor at frequent intervals.<br />• What is Motor controller?<br />A device that controls some or all of the following functions: starting, stopping,<br />overload protection, over current protection, reverses, changing of speed sequence<br />control and running/jogging.<br />• What is Motor speed?<br />The shaft speed of the three-phase squirrel cage motor is determined by the<br />frequency of the supply voltage and the number of poles in the motor. A two-pole<br />motor runs at a speed of 3000 rpm on 50 cycles per second.<br />rpm = cycles per second x 60 - slip<br />Poles<br />(Where slip is the difference between the speed of the rotating magnetic field and the<br />speed of the rotor.)<br />• Why Over current protection used?<br />A fusible disconnect or circuit breaker used to protect the branch circuit conductors,<br />control devices and the motor from grounds and short circuits. the over current<br />protection device must be capable of carrying the starting current to exceed 400% of<br />the motor full load current.<br />• What is Overload?<br />Any excessive amount of current drawn by the motor is called overload. Overloads<br />on a motor may be mechanical or electrical.<br />• What is Plugging?<br />The instant reversal of motor is called plugging. Damage to the driven machinery<br />can be result if plugging is applied improperly.<br />• What is Sequence control?<br />Question and answers Electrical Maintenance Unit<br />- 314 -<br />The control of separate motors to operate in a predetermined pattern.<br />Question and answers Electrical Maintenance Unit<br />- 315 -<br />• What is Service factor?<br />The amount of overload that may be permitted without causing significant<br />deterioration of the insulation on a motor. For example, if a 10 hp motor has a<br />service factor of 1.15, the motor can be safely be subjected to an 11.5 hp load.<br />• What is Starting current or Locked rotor current?<br />The current flow in the motor at the instant of starting. This current can be 4-10<br />times the full load current of the motor. The most common locked rotor current is<br />about 6 times the full load current. Such a motor will start with a 600% overload.<br />• What is Torque?<br />The twisting force produced by the motor is called torque. Its unit is in foot-pounds<br />(ft-lb.), torque is related to horsepower by the following formula.<br />Torque = horsepower * 5252<br />Revolution per minute (rpm)<br />• Write details MCC construction.<br />a) MCC are made up of sheet steel enclosure, indoor floor mounting and free<br />standing, Dust and vermin proof, modular type and of double front and single<br />front (X1, Y1).<br />b) Degree of protection is IP 50 as per IS 2147.<br />c) 0.9 * 0.8 * 2.4 meters size (double front and single front) and top entry of cables.<br />d) 0.9 * 0.6 * 2.4 meters size (single front) and bottom entry of cables.<br />e) Parts are incoming panel, Cable entry, TB compartment, MCC module<br />compartment.<br />f) MCC modules are fully drawn-out type.<br />g) Main buses are horizontally mounted and vertical buses are connected to MCC<br />cells.<br />h) Stab – in contacts are used for power and wipe – in contacts are used for control<br />circuits.<br />i) CT and PT are used for current and voltage measurements.<br />j) 3φ indication lamps are provided for identification.<br />k) Voltage meter and ammeter are provided.<br />l) Panel space heater and emergency push button key operated are provided.<br />m) Control building and SRPH MCC are safety related and SB, TB, RAB, CWPH,<br />DM Plant MCC’s are non safety related.<br />• What is the maximum load on MCC?<br />3 Phase load upto 90 kW are fed by MCC.<br />Question and answers Electrical Maintenance Unit<br />- 316 -<br />• What are the types of MCC?<br />Type Application Protection<br />A Receptacles, Cranes, Elevators, Local control panels Fuse<br />B Locally controlled heaters Fuse, 49<br />C Remote controlled heaters Fuse, 49<br />D Remote controlled loads <29 kW (49 in control panel) Fuse<br />F For valve motors Fuse, 49<br />G Locally controlled loads <29 kW Fuse, 49<br />H Locally controlled loads >29 kW (RTM installed) Fuse, 49<br />I DG MCC<br />J Remote controlled loads <29 kW Fuse, 49<br />K Remote controlled loads >29 kW (RTM installed) Fuse, 49<br />L Barring gear motor MCC Fuse, 49<br />SP For F/M supply and PHT S/D cooling pumps CT, PT Used<br />• What is the operating life of bearings?<br />a) Continuous 24 hrs operation – 40000 to 50000 hrs.<br />b) Affected by load axially or radial.<br />c) Operating temperature.<br />• Give the 415 V MCC bus ratings and cable used.<br />All MCC bus bars are made of aluminium. Short time current is 50 kA/sec and<br />momentary rating is 105 kA.<br />Type Bus Rating Class Location Cable Used<br />K1 1000 A IV TB 111 Mts.<br />K2 1500 A IV TB 111 Mts.<br />L1 1000 A IV RAB 108 Mts.<br />L2 1000 A IV SB 106.5 Mts.<br />L3 1500 A IV TB 111 Mts.<br />M1 1000 A IV SB 106.5 Mts.<br />M2 1500 A IV TB 111 Mts.<br />N1 1000 A IV TB 111 Mts.<br />N2 1000 A IV RAB 108 Mts.<br />W1 1500 A IV CCW PH 98 Mts.<br />W2 1000 A IV DM plant 98 Mts.<br />P1 1500 A III CB 100 Mts.<br />P2 1500 A III CB 106.5 Mts.<br />Q1 1500 A III CB 100 Mts.<br />Q2 1500 A III CB 106.5 Mts.<br />X1 600 A III SRPH 100 Mts.<br />Y1 600 A III SRPH 100 Mts.<br />PMCC S 630 A II CB 106.5 Mts.<br />Question and answers Electrical Maintenance Unit<br />- 317 -<br />PMCC T 630 A II CB 106.5 Mts.<br />Question and answers Electrical Maintenance Unit<br />- 318 -<br />• What are the types of isolators used?<br />Isolator Range Loads<br />32 A ≤ 9.3 kW<br />63 A > 9.3 kW and ≤19 kW<br />125 A > 20 kW and ≤ 47 kW<br />250 A > 48 kW and ≤ 110 kW<br />400 A > 111 kW and ≤ 134 kW<br />600 A ≤ 310 kW<br />• What are the ranges of fuse used?<br />Fuse Range Loads<br />2 A 10 to 280 watt<br />4 A 340 to 440 watt<br />6 A 500 to 700 watt<br />10 A 1000 to 1500 watt<br />16 A 1.8 kW to 2.25 kW<br />20 A 3 to 4 kW<br />25 A 5 to 8 kW<br />32 A 9 to 9.3 kW<br />50 A 9.6 to 15 kW<br />63 A 16 to 19 kW<br />80 A 22 to 24 kW<br />100 A 25 to 36 kW<br />125 A 38.6 to 45 kW<br />160 A 48 to 67.5 kW<br />200 A 72 to 80 kW<br />250 A 85 to 90 kW<br />• What is use of fuse in electric circuit, what are the materials used for fuse and what<br />are their melting points?<br />Fuse is a weakest point in an electrical circuit, which breaks the circuit when<br />abnormal current more than it’s rating flows through it. It works on principle of joule<br />law (I2Rt). HRC fuse is filled with quartz powder to extinguish the arc generated in<br />breaking the circuit or when fuse blown.<br />Current rating is depends on the type of material, cross section area, length and size<br />of terminal (large size terminal dissipates more heat).<br />Formulae<br />H = I2Rt/J<br />R = ρl/a<br />a = d2π/4<br />Material Melting point in °C<br />Silver 1830<br />Copper 2000<br />Question and answers Electrical Maintenance Unit<br />- 319 -<br />Aluminium 240<br />Zinc 787<br />Tin 436<br />Lead 624<br />Question and answers Electrical Maintenance Unit<br />- 320 -<br />• What are the materials made of thermal overload relay? How OLR are selected?<br />The bimetallic materials are Invar and brass. These materials having the differential<br />coefficient of expansion. All bimetallic relays incorporate additional built in single<br />phasing protection.<br />The range of the relay provided for the feeders are such that the full load rating of the<br />feeders is comfortably within the range of the relay (range will be at Centre) except<br />in very minimum loads ranging from 0.1 – 0.16 A.<br />• What are the functions of arc chute?<br />To increase the speed of rise of arc by magnetic action.<br />It splits the arc by this arc resistance increases.<br />Diagnosing the arc by cooling.<br />• What are IP (ingressive protection) and IC?<br />IP means ingressive protection to the motor against the dust and water entry.<br />The first digit indicates protection against accidental contact with live or moving<br />parts (solid particles).<br />The second digit indicates protection against ingress of water, foreign bodies (liquid<br />particles).<br />IC means instrument cooling to the motor (type of cooling)<br />• What are the classes of AC motors?<br />Depends on phases<br />a. 1φ.<br />b. 3φ.<br />Depends on construction<br />a. Squirrel cage induction motor for fixed torque.<br />b. Wound rotor motor for variable torque.<br />Depends on voltage<br />a. LT motor - <200 kW.<br />b. HT motor - >200 kW.<br />Depends on torque and current<br />a. Class – A (Normal torque and normal starting current. E.g. Fractional motors.) .<br />b. Class – B (Normal torque and low starting current).<br />c. Class – C (High starting torque and low starting current. E.g. Double sq. cage motor)<br />d. Class – D (high starting torque and high starting current).<br />Depends on mechanical characteristics<br />a. Drip proof (IP 54). Safety against water or dust.<br />b. Splash proof.<br />c. TEFC (totally enclosed fan cooled).<br />d. TEOV (totally enclosed open ventilated).<br />e. TETV (totally enclosed tube ventilated. Principle is thermosymphony E.g. - CEP).<br />f. Explosion proof.<br />Question and answers Electrical Maintenance Unit<br />- 321 -<br />• What is the PI value required for motors?<br />For class F insulation >2 and for class B insulation 1.5 to 2.<br />Question and answers Electrical Maintenance Unit<br />- 322 -<br />• What are the enclosures used for HT and LT motors?<br />LT motors (<200 kW)<br />a. Open drip proof.<br />b. TEFC.<br />c. Totally enclosed air over type.<br />HT motors (>200 kW)<br />a. Open drip proof.<br />b. Weather protected I<br />c. Weather protected II<br />d. Totally enclosed water-cooled.<br />e. Totally enclosed pipe ventilated.<br />• What are the causes of motor failure?<br />a. Corrosion or rust.<br />b. Excessive moisture (winding IR low and bearing lubrication loss).<br />c. High ambient temperature.<br />d. Poor ventilation.<br />e. Inadequate lubrication.<br />f. Misalignment.<br />g. Oil and dirt.<br />h. Excessive starts and repetitive surges.<br />i. Persistent over loads.<br />j. Shaft currents (bearing pitting).<br />k. Mis application.<br />l. Manufacture defect or wrong design.<br />m. Deterioration with age.<br />n. Maintenance improper.<br />• What are the effects of excessive starts and repetitive surges?<br />Repetitive surges may give impact to the insulation of the motor and dielectric<br />capability of the motor.<br />Excessive starts may subject stator winding to high current for more time.<br />Subsequently in HT motor due to High Mass rotor bar and rotor short ring may loose<br />or fail. Bearing also may damage.<br />• What are the effects of broken rotor bars and broken shaft parts?<br />Broken rotor bars<br />a. High stator current and over heat of stator winding.<br />b. More harmonic currents in end parts.<br />c. High vibration.<br />Broken shaft or parts<br />a. Stator winding loose bracing.<br />b. Rotor high vibration and bearing vibration.<br />c. Frame vibration and more harmonics in side bands.<br />• Give the relation between current and temperature in motors.<br />Question and answers Electrical Maintenance Unit<br />- 323 -<br />a. Winding temperature is proportional to square of the current.<br />b. 10% increase in current gives 30% increase in temperature.<br />c. 10°C rise in temperature makes 50% life reduced.<br />Question and answers Electrical Maintenance Unit<br />- 324 -<br />• What are the effects of imbalance stator winding resistance?<br />If the stator winding resistances are imbalance phase to phase give pulsating fluxes<br />and pulsating torque on rotor and vibration may increase. For accurate resistance<br />measurement Kelvin Bridge is used.<br />• What reflects the change in speed?<br />a. Supply frequency may vary the speed of the motor.<br />b. Load on the motor may vary the speed of the motor.<br />• What is use of BORESCOPE inspection?<br />BORESCOPE inspection method makes it easy to observe the end winding condition<br />of the motor. In this inspection winding ties, loose coils, dust etc can be observed.<br />• What you mean by CRAWLING and COGING?<br />Crawling<br />The motor fails to rotate at rated speed or motor rotates at … or 20% speed is called<br />motor crawling. This may be due to system imbalance or more pulsating torque.<br />Coging<br />Motor fail to start atoll is called motor coging.<br />• Why motor starting current is high compared to transformer charging current?<br />Transformer charging current is only 1% and that of motor starting current is 30 to<br />40%. Because of air gap between stator and rotor. If the air gap is more load taking<br />capacity increases and if air gap is less the load taking capacity reduces.<br />• State construction details of the motor.<br />Stator or rotor core<br />Built from high quality low loss silicon steel laminations and flash enameled on both<br />the sides made up of close-grained alloy cast iron.<br />Rotor conductor<br />Heavy bars of copper or aluminium alloy.<br />Stator<br />Copper conductor.<br />• What is the use of making rotor skewing?<br />1. To run motor quickly by reducing magnetic hum.<br />2. It reduces locking tendency with the stator.<br />• Why under voltage tripping of motor is incorporated in motor feeder breakers?<br />The under voltage can occur in case of bus fault. If the motors are kept connected<br />they will feed the fault which may cause the damage. Due to the back feeding from<br />the motor the motor will slow down very fast. Hence process system will come to<br />Question and answers Electrical Maintenance Unit<br />- 325 -<br />halt very fast. (In case pht motor will not rotate for the designed 3 minute period in<br />case of bus under voltage).<br />Question and answers Electrical Maintenance Unit<br />- 326 -<br />• What are the problems in station operation due to grid under voltage?<br />a) All the HT motors overloaded.<br />b) VAR load increases on generator leads to heating up of rotor<br />c) Stator current increases for same power export leads to stator over heating.<br />• What are the problems in station operation due to grid under frequency?<br />a) Turbine having under frequency limitation, house load happen if < 48 Hz<br />b) Due to under frequency PHT flow reduces, therefore reactor power reduces,<br />generator power reduces<br />c) If frequency is less than 48 Hz DG cannot be synchronised to grid, therefore DG<br />kept on isolation running<br />d) GT overfluxing.<br />• What is the difference between fixed trip and trip free?<br />Fixed trip: Breaker will trip only after closing even if trip impulses are existing.<br />Trip free: Breaker is free to trip at any position.<br />• What is the making current capacity of a 3-phase breaker as derived from its<br />symmetrical breaker capacity?<br />Making capacity = 2.55 times symmetrical breaking capacity.<br />• Why intermediate contacts in English electric breaker?<br />To prevent even slightest arcing on main contacts.<br />• Where preloaded ball bearings used?<br />If more vibration exists even when machine is not in running conditions.<br />• Why do we grease the bearings?<br />a) Grease lubricant gives good protection against ingress of moisture and dirt into<br />motor.<br />b) Easy to seal against leakage of grease into motor compared to oil.<br />c) Low friction torque at starting.<br />• Which bearings preferred for all large power motor?<br />Plain bearings<br />• Which is more dangerous alkali or acid?<br />It especially exposed alkali is more dangerous, use boric acid powder solution<br />immediately.<br />• What FCN was implemented to avoid reactor trip on 220V-DC failure of PHT and<br />PPP breakers?<br />Question and answers Electrical Maintenance Unit<br />- 327 -<br />The breaker close position supervision relay VAA 21 is changed by VAJC type,<br />contacts position do not change if 220V-DC is lost now.<br />Question and answers Electrical Maintenance Unit<br />- 328 -<br />• Where oil lubrication is preferred over grease lubrication?<br />a) Determined by speed and operating temperature.<br />b) Oil lubrication recommended.<br />c) When speed and temperature is high.<br />d) When heat to be conducted away from bearing.<br />e) When adjacent machine components are oil lubricated.<br />f) High viscous oil for low speed machine, low viscous oil for high speed machine.<br />g) At temp<125 ºC, synthetic oils recommended.<br />• What are the causes of failure of bearings?<br />a) Faulty mounting<br />b) Faulty lubrication<br />c) Foreign matter in lubrication<br />d) Water in the bearing arrangement<br />e) Vibration<br />f) Inoccurrences of form of shaft or housing seating.<br />g) Passage of electric current.<br />h) Metal fatigue.<br />• What does the bearing number mean?<br />7318 7 = single row angular contact ball bearings<br />3 = width of race<br />18 = 18 x 5 = 90 mm bore diameter.<br />6310 6 = single row deep groove ball bearings.<br />3 = width of race<br />10 =10 * 5=50 mm dia<br />• What is the purpose of static starter? How current setting adopted?<br />The static starter limits the starting current of the motor to 2.5 times the motor rated<br />current instead of 6 times the rated current. If the motor is directly on UPS, the UPS<br />fuse will blow, since the UPS cannot supply so much starting current. Hence the<br />static starter is set to limit the starting current. This is achieved by firing angle<br />control of back to back thyristor.<br />• What is the speciality of the inverter output transformer? Why it is provided?<br />a) This eliminates all 3rd harmonics in the output voltage.<br />b) Solid earthed neutral is required for the inverter output, hence the interconnected<br />star winding is essential.<br />c) The primary has to be star (not delta), since 3 separate inverters operates on<br />isolated primary winding.<br />d) Delta connection will cause circulating current between inverters during<br />unbalanced faults. The inverters cannot withstand this.<br />Question and answers Electrical Maintenance Unit<br />- 329 -<br />Question and answers Electrical Maintenance Unit<br />- 330 -<br />• Both silver and copper oxidise in air. Then why copper contacts are silver-plated?<br />The silver plating avoids the oxidation of copper, especially in outdoors. Silver<br />oxidises very readily. But its oxide is a good electrical conductor but copper oxide<br />produce a film of insulation.<br />• When auto transfer is effected?<br />a) When any one module trips<br />b) When overload exceeds 175% for more than 40 msec. is existing.<br />c) When UPS output voltage varies beyond 415v +/- 10%<br />• When static bypass is fired? Why static bypass is required?<br />For the same above 3 conditions, static bypass is also simultaneously fired along<br />with a closing impulse.<br />• When the static bypass is blocked?<br />When the phase error is more than 20º.<br />• What is phase lock mode?<br />The inverter continuously follows the frequency and phase angle of classIII bus<br />supply.<br />• What is the difference between a contactor and a breaker?<br />Contactor is not designed to open on short circuit condition (fuse will take care of<br />this situation). Breaker is having complicated mechanism for closing and tripping.<br />• What is the difference between isolator and contactor?<br />Contactor is used for on load operation. Because they are fast acting devices. They<br />posses arc chamber and arc chutes. Arc chamber and arc chute make it easy to<br />extinguish the arc produced during on load operation.<br />Isolator is off load devices. Because they are slow acting devices. The arc time is<br />more in slow acting devices and operated only in off load.<br />• What are the protections provided for motor feeder?<br />Ith - Thermal over load<br />I2S - Unbalance load<br />I0S - Earth fault protection<br />I1t - Stalling protection<br />I1Inst. - Short circuit protection<br />• What are the protections provided in PMCC circuit breakers?<br />1. IDMT O/C (CDG 34).<br />2. IDMT E/F (CDG 11).<br />Question and answers Electrical Maintenance Unit<br />- 331 -<br />3. Under voltage (40% of 110V).<br />Question and answers Electrical Maintenance Unit<br />- 332 -<br />• What is requirement of having DG’s?<br />To establish class III supply when class IV fails.<br />Parallel operation with class IV 6.6 kV supply.<br />DG to DG parallel operation.<br />• What are the characteristics of CB, OLR and HRC Fuse?<br />CB characteristics (it is back up fuse)<br />Current<br />Fuse characteristics<br />Margin to avoid fuse operation OLR characteristics<br />During starting<br />Minimum fusing current<br />Staring current<br />Running current<br />Time<br />CB Protection<br />Fuse Protection<br />OLR Protection<br />I<br />Time<br />When CB is used the CB characteristics should be below the fuse, because the CB<br />should operate first and then fuse. Not vice-versa. Because CB is the main protection<br />or main breaking device.<br />• Why control transformer is earthed?<br />If it is not earthed grounding of control circuit at two different places can cause<br />bypassing of logics. In case of primary and secondary of the control transformer is<br />getting the main fuse will blow off. (If secondary is not grounded then 415v will be<br />superimposed in the control circuit during short circuit of primary and secondary<br />winding)<br />• When the fuse will take over?<br />When the current increases beyond 700% then the fuse will take over from the<br />thermal overload protection.<br />• What is interlocks provided for the valve MCC<br />Question and answers Electrical Maintenance Unit<br />- 333 -<br />a) Mechanical interlock, which will not allow the other contactor to close if one<br />contactor, is closed.<br />b) 42 auxiliary contacts are wired in the control ckt. 42-1 contact in 42-2 and 42-2<br />contact in 42-1.<br />Question and answers Electrical Maintenance Unit<br />- 334 -<br />• How to calculate the full load current of the motors?<br />If kW is given, full load current = 1.5 times kW rating.<br />If hp is given, full load current =2 times hp rating.<br />• What is the safety interlock provided in MCC cell?<br />The MCC cell door can't open if the cell is in on condition.<br />• How the fuse is selected?<br />Fuse rating should be 2.5 times the full load current.<br />• How will you improve the IR value of a motor?<br />By providing external heating. (By filament lamps)<br />By providing internal heating by applying the low voltage.<br />By circulating hot and dry air.<br />• Why 110V has been chosen in MCC cell?<br />To isolate control circuit from power circuit for Human safety at control circuit side.<br />• What is the purpose of DIODE across the interposing coils in PLC?<br />To dissipate the stored energy in magnetic field of the interposing coils (Free<br />wheeling action). If it is not provided the stored energy will affect the PLC card<br />circuit.<br />• What are the in-built protections provided in MCC cell?<br />Fuses for short circuit protection.<br />OLR for over load and single phasing protection.<br />Electrical and Mechanical interlock in valve cell against short circuit.<br />• What is the plugging of an induction motor?<br />It is an electrical braking of an induction motor by sudden reversal of phase<br />sequence.<br />• Why CT operated over load relay is using for loads of high acceleration time upto 30<br />seconds? How it getting back?<br />The saturable current transformers linearly transforms the current upto twice the set<br />current, but above this value the transformer core gets saturated and the secondary<br />current is proportionally less. Thus these relays permit heavy starting conditions of<br />motors and offer dependable protection against overload.<br />When current reduces the core gets de-saturated, as material design is such.<br />• How many earthing should be done for motors? Why?<br />Question and answers Electrical Maintenance Unit<br />- 335 -<br />Two. For reliability.<br />• What is the significance of frame size of motor?<br />In order to make practical choice, interchangeability and large scale production<br />possible.<br />Question and answers Electrical Maintenance Unit<br />- 336 -<br />• What is polarisation index?<br />a) It is defined as a ratio of 10 minutes resistance value to 1-minute resistance value.<br />b) It gives a quantitative information about the insulation with respect to moisture,<br />dirt and other contamination.<br />c) A PI value of less than 1.0 indicates a need for immediate reconditioning.<br />• Why megger value of 1 minute is less than 10-minute value?<br />After 10 minutes the high voltage applied make the molecules such a way that<br />stabilised in a good insulation. If insulation is weak it leads to more leakage current<br />due to high potential.<br />• What is the classification of duty of rotating electrical machines?<br />S1 – Continuous operation at rated load (MCR) in 40 °C<br />S2 – Short time operation (STR) for 5 minute or 15 minutes or 30 minutes.<br />S3 – Intermittent periodic operation (resting and loading e.g. cranes, lifts etc)<br />S4 – As for S3 but with starting<br />S5 – As for S3 with electric braking<br />S6 – Continuous cyclic operation.<br />• What should be the value of insulation resistance of induction motor?<br />In Rm = kV + 1 M OHMS.<br />Insulation resistance of any electrical machine (motor or generator) should be above<br />0.5-M ohms in all cases.<br />• What are the classes of insulation?<br />Y – 90οC (max) cotton, silk, paper, wood without oil impregnation<br />A – 105οC Materials of class Y impregnated with natural resins,insulating oils.<br />E – 120οC Synthetic resin enamels, cotton and proper laminations.<br />B – 130οC Mica, glass fibre, asbestos with suitable bonding substance.<br />F – 155οC Class B with more thermally resistant bonding materials.<br />H – 180οC Glass fibre and asbestos, mica with silicon resins.<br />C – >180οC Mica, ceramics, glass, quartz and asbestos without binders.<br />• What are the checks on the motor during the preventive maintenance?<br />IR Value<br />Resistance and Inductance measurement<br />PI value (should > 1.0)<br />• What are the tests pressures used in lyra contact testing?<br />125 A - 3 kg<br />250 A - 5 kg.<br />Question and answers Electrical Maintenance Unit<br />- 337 -<br />Question and answers Electrical Maintenance Unit<br />- 338 -<br />• What are the causes of motor vibration?<br />a) Broken rotor.<br />b) Slacked stator core.<br />c) Slacked rotor core.<br />d) Rotor winding unbalance.<br />• What are the causes for motor high current?<br />a) High frequency (51 Hz - 105% current)<br />b) Low frequency (48 Hz – 102% current)<br />c) High voltage<br />d) Low voltage<br />e) Mechanical over load<br />• What are the causes for motor unbalance current?<br />a) Loose connection<br />b) Voltage unbalance<br />c) Turns short circuit<br />• What are the sources of 240 V AC class I supply? What are the functions of each<br />part of UPS?<br />Six sources.<br />Three 20 kVA UPS for safety related loads.<br />Two 60 kVA UPS for non-safety related loads.<br />One 60 kVA UPS as a standby to safety related loads.<br />These all UPS are back up by 220V DC batteries.<br />Rectifier<br />This converts AC to DC supply for inverter.<br />Functions<br />1. Produces DC voltage.<br />2. Supplies trickle charge to batteries.<br />3. Full load boost charge capacity.<br />Inverter<br />This converts DC to AC supply for loads.<br />20 kVA inverter is transistor based and 60 kVA inverter is thyristor based.<br />Static switch.<br />To take stand by UPS into service.<br />Manual bypass<br />To take main UPS to maintenance by putting stand by UPS into service.<br />• What is station Black out condition?<br />Question and answers Electrical Maintenance Unit<br />- 339 -<br />Simultaneous failures of class IV and class III supply is called Station Black out. In<br />this condition class II power UPS will feed the necessary loads for a 30 minutes of<br />duration. After that supplementary control room (SCR) 5 kVA UPS is used for<br />secondary shut down system (SSS) ion chamber amplifier.<br />Question and answers Electrical Maintenance Unit<br />- 340 -<br />220 kV SYSTEM<br />• What is meant by Dielectric strength?<br />The maximum electrical potential gradient that a material can withstand without<br />rupture usually specified in volts/millimeter of thickness. This also has known as<br />electric strength.<br />• Give switchyard specification.<br />1. Type : Out door.<br />2. Scheme : Double main bus bar with bypass switching scheme is provided.<br />This allows maintenance of one bus or one CB without interruption.<br />3. Normal voltage : 220 kV.<br />4. Rated voltage : 245 kV (400 kV)<br />5. Impulse voltage : 1050 kV (peak)<br />6. One-minute level : 460 kV (rms.)<br />7. Dynamic current capacity: 102 kA (peak) and 40 kA for one sec.<br />8. Rated current capacity : 2000 A for main and 1600 A for feeder bus.<br />9. Clearances : Phase to earth – 2100 mm.<br />Phase to phase – 2100 mm.<br />Phase to ground – 5500 mm.<br />Sectional clearance – 4300 mm.<br />Creepage clearance – (Total) 5600 mm.<br />– (Protected) 2800 mm.<br />10. Maximum temperature rise above ambient - 45°C.<br />11. CB – SF6<br />12. Isolator – motor operated rotating type.<br />13. Number of bays – 16 Nos.<br />• Give the details of switchyard 220 kV CB, Isolator, CT, CVT and lightning arrestor.<br />220 kV SF6 Circuit Breaker<br />1. Make – ABB<br />2. Air pressure blocking a. Close Block – 17.3 bar.<br />b. Open block – 16.7 bar.<br />c. Auto reclose block – 19 bar.<br />3. SF6 pressure block a. Alarm – 5.2 bar.<br />b. Rated – 6 bar.<br />c. Limit – 5 to 6 bar.<br />d. Open block – 5 bar<br />4. Weight of gas / pole : 20 kgs.<br />5. Closing time : 130-milli sec.<br />6. Method of closing : Electro-pneumatic.<br />7. Compressor pressure : 20.5 kg/cm2.<br />Question and answers Electrical Maintenance Unit<br />- 341 -<br />Isolator<br />1. Type : High-pressure pressure relieving isolator (HPPR) central pole double<br />break.<br />2. CB and Isolator clearances : Phase to Phase – 4500 mm.<br />Phase to earth – 2300 mm.<br />Question and answers Electrical Maintenance Unit<br />- 342 -<br />Current transformer<br />1. Make : TELK made hermetically sealed.<br />2. Type : Single pole dead tank.<br />Capacitor voltage transformer (CVT)<br />This is capacitive potential divider and inductive medium mineral oil sealed.<br />ABB. make 245 kV/110 √3 V.<br />3 cores for metering and protection.<br />Lightning arrestor<br />Type : WS surge arrestor of ZODIVER type and SMX style.<br />Gapier zinc oxide arrestor. Multi unit construction for transport, storage and erection.<br />Rated voltage : 216 kV rms.<br />Operating voltage : 184 kV rms.<br />• For a fault in switchyard lightning arrestor, what protection will act?<br />Bus bar differential protection.<br />• What is the purpose of the CVT?<br />To provide synchronising signal<br />To provide voltage indication<br />To facilitate the carrier communication<br />• What is the purpose of wave trap?<br />Carrier communication signals are sent through the lines. These are high frequency<br />signals. This signal should be prevented from entering the switchyard. The wave trap<br />is LC ckt, which is tuned for 50 Hz. Since it is connected in series with the line it<br />will effectively block the carrier signal entering into the switchyard.<br />• What is the purpose of lightning arrestor?<br />Due to lightning and switching surges high voltages are induced in the lines. If<br />equipment’s. Connected is subjected to this high voltage the insulation will fail. In<br />order to avoid the failure of insulation the LA is used. When the la is subjected to<br />high voltage it will conduct and discharge the current to the earth.<br />(It will divert the over voltages to earth and protect the substation)<br />• What is meant by restriking voltage?<br />The high voltage that will appear across the contact just after the quenching of the<br />arc is called restriking voltage.<br />• What does switching surges mean?<br />Question and answers Electrical Maintenance Unit<br />- 343 -<br />When a line is switched on high voltage will appear on the line due to its inductance<br />and capacitance. This voltage is known as switching surges.<br />Question and answers Electrical Maintenance Unit<br />- 344 -<br />• What is the purpose of compressed air in SF6 breaker?<br />This used for drive for opening and closing of the contacts.<br />(Arc quenching is taken care by SF6 gas)<br />• Why switchyard is located indoors of coastal plants?<br />Saline atmosphere will deposit on the insulators causing its flashover. The building<br />kept under positive pressure compared with outside thus preventing the (saline) air<br />entering from outside to inside the building.<br />• Why disc insulators grooved at bottom?<br />To increase the creepage distance, reduce the chances of flash over.<br />• How cap and pin attached to insulator?<br />By cementing.<br />• What is the material of cap, pin, and insulator?<br />Cap = galvanised cast iron<br />Pin = forged steel pin<br />Insulator = porcelain.<br />• Why insulators are glazed?<br />If not glazed, it will absorbs water, resistance comes down, leakage current through<br />porcelain, temperature increases till porcelain is puncture<br />• What is the station ground resistance?<br />Less than 0.5 ohms.<br />• What is the various design of CT's in switchyard?<br />Bus coupler CT's- live tank design 2000-1000A/1A<br />All other CT's- dead tank design 800-600-400A/1A - lines and GT.<br />125A/1A – SUT<br />• Advantage of CVT over EMPT.<br />Used as coupling capacitors for PLCC.<br />• What are the main parts of 220 kV Circuit Breaker?<br />Pole column filled with SF6<br />Pneumatic drive system with compressed air circuit<br />Control cubicle unit<br />• What is the type of 220 kV circuit breaker?<br />220 kV, SF6 breaker, single pole, puffer type.<br />Question and answers Electrical Maintenance Unit<br />- 345 -<br />• What are the levels of SF6 gas in 220 kV breaker and their significance?<br />7 kg/cm2 - normal pressure<br />5.2 bar - alarm<br />5.0 bar - closing/tripping operation blocked.<br />Question and answers Electrical Maintenance Unit<br />- 346 -<br />• What is the difference between circuit breaker and isolator?<br />Isolator is a disconnecting switch which is not having the making and breaking<br />capacity.<br />Bus coupler - 2000A<br />Feeders - 1600 A<br />• What is the purpose of ground switch?<br />To discharge the trapped electrical charges to ground to give complete isolation.<br />(To discharge the residual potential)<br />• What is the type of Lightning Arrestor?<br />Station type, heavy duty, gap less zinc oxide.<br />• What is the purpose of grading ring?<br />This assembly is provided to have uniform voltage gradient.<br />• What are the properties of SF6 gas?<br />Physical properties<br />1. Colourless<br />2. Odorless<br />3. Non-toxic. Pure SF6 gas is not harmful to the health.<br />4. Non-inflammable.<br />5. Density- more gas density, 5 times that of air at 20°C and at atmospheric<br />pressure. The gas starts liquefying at certain low temperature. The temperature of<br />liquefaction depends on pressure. At 15 kg f / cm2 the gas starts liquefying at<br />10°C. Hence this gas is not suitable for high pressures >15 kg f / cm2<br />6. The heat transferability of SF6 gas is 2 to 2.5 times that of air at same pressure.<br />Hence for equal conductor size the current carrying capacity is relatively more.<br />Chemical properties<br />1. Stable upto 500°C.<br />2. Inert gas due to the chemical inertness. The life of the metallic parts, contacts is<br />longer in SF6 gas. The components do not get oxidised or deteriorated. Hence the<br />maintenance requirement is reduced. However moisture is very harmful to the<br />properties of the gas. In the presence of the moisture, hydrogen fluoride is formed<br />during arcing which can attract the metallic and insulating parts in the circuit<br />breaker.<br />3. Electro negative gas – Ability of an atom to attract means carrying a negative<br />electric charge.<br />These advantages offer increased safety, reduction in size, weight, noiseless<br />operation, easy installation, handling and maintenance.<br />Question and answers Electrical Maintenance Unit<br />- 347 -<br />• What are the protections are there for BUSBAR?<br />Instantaneous over current protection<br />Bus bar differential protection<br />Local breaker back up protection<br />Question and answers Electrical Maintenance Unit<br />- 348 -<br />• What are the protections are there for lines?<br />Directional earthfault protection<br />Directional over current protection<br />Local breaker back up protection<br />Pole discrepancy<br />Main protection (distance protection)<br />Directional OverCurrent Relay For Line-1&amp;2<br />MICOM-P127<br />(This relay is in addition to the existing electro-mechanical directional O/C relay)<br />Setting Details<br />CTR= 600/1A<br />VTR= 220kV/110V<br />Directional Over Current Setting<br />Description Symbol in Relay Set value LED Indication<br />IDMT Directional over<br />current (Stage#1)<br />Secondary 1.33Amps Primary<br />(800Amps) TMS = 0.1 (67ABC)<br />Directional<br />O/C(Stage#2)<br />Secondary 3.33Amps<br />Primary (2000Amps)<br />Instantaneous.<br />(67ABC)<br />Directional<br />O/C(Stage#3)<br />Secondary 5Amps<br />Primary (3000Amps)<br />Instantaneous.<br />(67ABC)<br />Directional Earth Fault Current Setting<br />Description Symbol in Relay Set value LED Indication<br />IDMT Directional Earth<br />Fault current (Stage#1)<br />Secondary 0.2Amps Primary<br />(120Amps) TMS = 0.1 (67N)<br />Directional Earth Fault<br />(Stage#2)<br />Secondary 4Amps<br />Primary (2400Amps)<br />Instantaneous.<br />(67N)<br />Directional Earth Fault<br />(Stage#3)<br />Secondary 6Amps<br />Primary (3600Amps)<br />Instantaneous.<br />(67N)<br />Question and answers Electrical Maintenance Unit<br />- 349 -<br />MICOM-P127<br />Directional OverCurrent Relay For Line-3&amp;4<br />Setting Details<br />CTR= 800/1A<br />VTR= 220kV/110V<br />Directional Over Current Setting as on 07/09/2002<br />Description Symbol in Relay Set value LED Indication<br />IDMT Directional over<br />current (Stage#1)<br />Secondary 1.0Amps Primary<br />(800Amps) TMS = 0.2 (67ABC)<br />Directional<br />O/C(Stage#2)<br />Secondary 0.94 Amps<br />Primary (750Amps)<br />Instantaneous.<br />(67ABC)<br />Directional<br />O/C(Stage#3)<br />Secondary 2.5Amps<br />Primary (2000Amps)<br />Instantaneous.<br />(67ABC)<br />Directional Earth Fault Current Setting<br />Description Symbol in Relay Set value LED Indication<br />IDMT Directional Earth<br />Fault current (Stage#1)<br />Secondary NOT USED<br />Primary TMS = (67N)<br />Directional Earth Fault<br />(Stage#2)<br />Secondary NOT USED<br />Primary<br />Instantaneous.<br />(67N)<br />Directional Earth Fault<br />(Stage#3)<br />Secondary NOT USED<br />Primary<br />Instantaneous.<br />(67N)<br />Question and answers Electrical Maintenance Unit<br />- 350 -<br />220KV SWITCH YARD LINES<br />DESCRIPTION LINE-01 LINE-02 LINE-03 LINE-04<br />REMARKS<br />LINE LENGTH<br />IN KM<br />13 16 62 62<br />CT RATIO 600/1 A 600/1 A 800/1 A 800/1 A<br />01 DISTANCE<br />RELAY<br />SETTINGS.(21)<br />Relay character QUAD QUAD LENT LENT<br />K1 1 2 4 4<br />K2 0 0 0.8 0.8<br />K3 32 32 N/A N/A<br />K4 0 1 4 4<br />K5 0.7 0.4 0.3 0.3<br />K6 0 0 0.02 0.02<br />K11 1 1 1 1<br />K12 0.6 0 0.3 0.3<br />K13 0.08 0.04 0.02 0.02<br />K14/24 1 1 1 1<br />K15 1 1 1 1<br />K21 4 2 2 2<br />K22 0.5 0.6 0.5 0.5<br />K31 6 3 4 4<br />K32 0.9 0.9 0.5 0.5<br />K33 1 1 1 1<br />K35 1 1 2 2<br />K36 0 0.2 0.7 0.7<br />K37 1 0.5 0.25 0.25<br />A/b N/A N/A 1 1<br />Z-2 TIME(m sec) Inst. Inst. 400 400<br />Z-3 TIME(m sec) 120 120 800 800<br />L#2- Z-2 &amp; Z-3<br />changed on<br />18/6/02 &amp; L#1 on<br />21/06/02<br />Tp ALL LEFT ALL LEFT ALL LEFT ALL LEFT<br />Td ALL LEFT ALL LEFT ALL LEFT ALL LEFT<br />SW-1 RIGHT RIGHT RIGHT RIGHT<br />SW-2 LEFT LEFT LEFT LEFT<br />SW-3 RIGHT RIGHT RIGHT RIGHT<br />SW-4 RIGHT RIGHT RIGHT RIGHT<br />SW-5 RIGHT RIGHT RIGHT RIGHT<br />SW-6 RIGHT RIGHT RIGHT RIGHT<br />SW-7 LEFT LEFT LEFT LEFT<br />SW-8 LEFT LEFT LEFT LEFT<br />SW-9 RIGHT RIGHT RIGHT RIGHT<br />Z-1 LEFT LEFT LEFT LEFT<br />Z-2 LEFT LEFT LEFT LEFT<br />Z-3 RIGHT RIGHT RIGHT RIGHT<br />ANGLE (Ph-Ph) 80 80 85 85<br />ANGLE (Ph-N) 80 80 75 75<br />TEST OPTION 0 0 0 0<br />Question and answers Electrical Maintenance Unit<br />- 351 -<br />DESCRIPTION LINE-01 LINE-02 LINE-03 LINE-04 REMARKS<br />2 DIRECTIONAL<br />OVER CURRENT<br />(67 A,B,C.)<br />CT RATIO 600/1 A 600/1 A 800/1 A 800/1 A L#2 Settings<br />changed on<br />18/6/02 &amp; L#1 on<br />21/6/02<br />PSM 1.25 (750A) 1.25 (750A) 1.0 (800A) 1.0 (800A)<br />TMS 0.1 0.1 0.2 0.2<br />High set 5A 5A 5A 5A<br />L#2 Settings<br />changed on<br />18/6/02 &amp; L#1 on<br />21/6/02<br />3 DIRECTIONAL<br />EARTH FAULT<br />(67N)<br />PSM 0.2A 0.2A 20%(0.2A) 20%(0.2)<br />TMS 0.1 0.1 0.225 0.225<br />High set 2A 2A 400%(4A) 400%(4A)<br />L#2 Settings<br />changed on<br />18/6/02 &amp; L#1 on<br />21/6/02<br />4 LBB<br />PROTECTION<br />(50Z)<br />PSM 0.2 0.2 0.2 0.2<br />KNOB 1 1 1 1<br />2/50Z 0.25 sec 0.25 sec 0.25 sec 0.25 sec<br />5 POLE<br />DISCRIPENCY<br />(47)<br />2/47T 0.1 sec 0.1 sec 0.1 sec 0.1 sec<br />6 ISOLATOR<br />PARALLEL(29)<br />TIMER(29) 25 sec 25 sec 25 sec 25 sec<br />7 POWER RELAY<br />(32)<br />PSM N/A N/A 10 ma 10 ma<br />TIME N/A N/A 0 0<br />8 INST, OVER<br />CURRENT<br />RELAY<br />(50A,,B,C.)<br />KNOB N/A N/A 94% 94%<br />2/50 ABC N/A N/A MIN. sec MIN. sec<br />Question and answers Electrical Maintenance Unit<br />- 352 -<br />BATTERIES<br />• What are the disadvantages of the maintenance free battery?<br />The life of battery is only five years.<br />The state of charge of a battery not knowing by the specific gravity of a battery. We<br />can know by voltage only.<br />• What are the problems of hydrogen concentration?<br />If the concentration of hydrogen more than 4% and less than 74% explosion<br />problems will be there. Therefore the concentration of hydrogen is restricted to less<br />than 1% by air changer ventilation system.<br />• Why lead acid battery requires so much large initial charging?<br />Initially for a new battery, negative plate will be PbO instead of Pb. To convert all of<br />them back to Pb, we need so much prolonged initial charging.<br />• What are the protections adopted in UPS or PMCC supply?<br />LV (incomer) or UPS input<br />CTZM, Over current, Short circuit protections<br />PMCC S or T input or UPS output<br />Under voltage (27), 51RYB, 51N protections.<br />• Write chemical equation for lead acid cell.<br />PbO2 + H2 SO4 􀃆 PbSO4 +2 H2 (during discharge)<br />Pb + SO4 􀃆 PbSO4 (during charge)<br />• Is the chemical reaction of plante cells same as tubular lead acid cells?<br />No. Plante cells having both electrodes are lead (Pb) only.<br />During charging, H2O 􀃆 H2 + O2<br />O2 react with Pb to form PbO2 + (+ve plate)<br />During discharge, Pb + c 􀃆PbSO4 (on -ve plate)<br />While PbO2 􀃆 Pb + O2 (on +ve plate).<br />That is converted back to lead.<br />Therefore PbSO4 formed only on -ve plate. That is sulphation problem are reduced<br />by 50%<br />• What are the advantages of plante type batteries?<br />Plante plate type batteries have longer life and can with stand rapid discharge.<br />• Why battery room should be located separately in a power station?<br />Possibility of battery explosion<br />Corrosive atmosphere by acid spray.<br />Fire hazard.<br />Question and answers Electrical Maintenance Unit<br />- 353 -<br />• What are the disadvantages of nickel cadmium battery?<br />Status of charge not known<br />Number of cells are more<br />Cost very high<br />Environmental protection agency considers cadmium as a hazardous material,<br />difficult to discard at the end of life.<br />• What are the effects of temperature in lead acid battery?<br />Higher electrolyte temp - ah capacity increases but life reduces.<br />Lower electrolytic temp - ah capacity reduces since chemical reaction rate reduces.<br />• Why ventilation is essential for Ni-Cad also?<br />Gases evolved H2 O2 can form explosive mixture.<br />• How H2 O2 generated in lead acid battery?<br />At end of charge, when most of the Pb is converted. H2 O2 generated from H2O. O2<br />appears as gas at positive plate. H2 at negative plate, i.e. gassing starts.<br />• Why current reduced after gassing?<br />Excessive gassing shortens the life of battery by scouring the active materials at the<br />surface of the plates.<br />• Why aged battery consumes more water?<br />As aging increases, antimony migrates to negative plate 􀃆 secondary cell reaction.<br />Therefore more charging current require 􀃆 more water consumption.<br />• What happens after aging?<br />Shedding of active material during charging.<br />Shedding increases with overcharging, heavy discharge, batteries short ckt.<br />• What is the other effect of low temperature?<br />When specific gravity decreases, acid freezing point increases, soon reached at low<br />temperature, volume increases container cracks.<br />• Why temperature correction required?<br />As temperature increases, specific gravity decreases. The hydrometer immersion is<br />more, showing lower readings. Therefore 10ºC raises, 7 points to be added,<br />corrected to get 27 ºC reading.<br />• What is meant by sulphation of the cell?<br />Question and answers Electrical Maintenance Unit<br />- 354 -<br />During the discharge lead sulphate is produced and during the charging the same is<br />converted back into lead and lead peroxide. If the cell is left under charged, lead<br />sulphate would form which will not reverse back into lead and lead peroxide during<br />charging. Due to this the cell will loose its original capacity.<br />Question and answers Electrical Maintenance Unit<br />- 355 -<br />• What you meant by shedding?<br />During the charging and discharging the active materials will undergo volumetric<br />changes. Due to this some of the material may not be retained with the parent<br />material and will shed and collected at the bottom of the container. This loss of<br />active material is called shedding. Due to this the cell will loose its capacity.<br />• Acid should be poured to water. Why is it so?<br />When acid and water is mixed lot of heat is generated. Hence there is chances of<br />splashing of the liquid. If water is poured to acid will splash causing injuries. In the<br />other case splashing will be of water with concentration of acid, which will not<br />hazardous as the other one.<br />• What types of lighting fittings are used in the battery room?<br />Flame proof acid resistant<br />• Why ungrounded 250V DC system adopted in our system?<br />The 250V DC system is feeding to some of the vital loads such as breaker control<br />etc. Even if one ground has occurred then also these controls should be available.<br />Continuous monitoring of ground current is employed to eliminate the by passing of<br />logic due to double ground.<br />• Why battery capacity limits to 20 minute?<br />Battery cost is more.<br />It is better to restore class 3 faster by DG set then putting large battery.<br />20 min, is enough to shutdown the unit safely.<br />• What are the main parts of lead acid battery?<br />a. Container<br />b. Lead dioxide positive plates<br />c. Lead negative plates<br />d. Post strap and seal assemble<br />e. Separators and retainers<br />f. Sulphuric acid electrolyte<br />g. Inter cell connector (lead plated copper)<br />• What are the different types of charging?<br />(Normally always) Float charging – 2.15V per cell<br />It maintains the battery fully charged condition during standby operation by<br />delivering a small amount of current to cancel the effect of battery natural selfdischarge.<br />Equalizing charging (2.7V/cell) once in 3 months<br />Question and answers Electrical Maintenance Unit<br />- 356 -<br />Recharge a battery capacity through recovering all useable active materials in the<br />cell plates.<br />Boost charging<br />Boost charging is a quick charging process, which is generally required, if the battery<br />is drained to a large extent.<br />Question and answers Electrical Maintenance Unit<br />- 357 -<br />• What is the double sulphate reaction?<br />Pb O2 + Pb + 2H2 SO4 ↔ 2PbSO4 +2H2O<br />During charging oxygen at positive plate and hydrogen at negative plate are<br />releasing.<br />• What are the effects of over charging?<br />Gassing<br />Heating<br />Loosening of plate active material<br />• What you mean by Drooping characteristics of charger?<br />When the charger is connected to excess load of charger rating the charger should<br />able to supply the load with out over loaded by maintaining the terminal voltage<br />within limit without over load trip. This called a drooping output voltage<br />characteristics.<br />• What are the effects of under charging?<br />Sulphation<br />Buckling of plates<br />• What are the effects of high temperature?<br />Gassing of electrolyte and evaporation<br />Service life is halved for every 8 deg increase above 25 deg.<br />• What are the effects of low temperature?<br />Increased electrolyte viscosity.<br />• What type of thermometer is used for acid batteries?<br />Alcohol type thermometer.<br />• What are the tests for battery?<br />Conduct test – to check the capacity batteries<br />Impedance test – to check the utilization of active materials.<br />• What are the functions of charger?<br />1. For initial charging.<br />2. For float charging.<br />3. For battery equalizing charging.<br />4. For battery boost charging.<br />5. To supply normal DC loads.<br />Question and answers Electrical Maintenance Unit<br />- 358 -<br />• What is the rating of switchyard battery charger and batteries?<br />Switchyard is having Float cum boost charger of 282 V DC maximum and 100<br />Amps rated charger of six-pulse full wave thyristerised controlled rectifier.<br />Batteries are tubular 220 V DC. +ve plate is made up of low antimory lead selenium<br />(Pb) and –ve plate is made up of paste plate type (O2). Container or tube is made up<br />of polyester and glass fibre.<br />1. Momentary load 160 A / minute.<br />2. Continuous load 40 A / hour.<br />3. Cell voltage 1.8 V DC and total number of cells are 106 in battery bank.<br />4. Float voltage 2.16 V per cell to 2.18 V per cell.<br />5. Maximum system voltage is 106 * 2.18 = 242 V DC<br />Battery rated for 224 A for one minute or 80 A for 60 minutes.<br />Specific gravity 1110 ± 5 and specific gravity after 10 hrs discharge is 1150 ± 5.<br />• What are the protections provided in charger?<br />1. Over load (49).<br />2. Over voltage (59).<br />3. Short-circuit (3250 Amps).<br />4. Phase sequence and phase fail.<br />5. di/dt and dv/dt protection.<br />Question and answers Electrical Maintenance Unit<br />- 359 -<br />CIRCUIT BREAKER<br />• Give circuit breaker nameplate details of 6.6 kV, and 415 V.<br />6.6 kV (SF6 circuit Breaker) 415 V (Air Circuit Breaker)<br />Type HPA12/1240C(Tr./ PM) 812 (MCC/UPS)<br />HPA12/2040C(Tr./ PM) 610 (Tie/PM/MCC)<br />3037 (B/c, Incomer)<br />Standard IEC 56<br />Rated voltage 12 kV (6.6 kV) 415 V<br />Insulation level 28 / 75 kV 660 V<br />Rated current 1250A/2000A 1600A/1000A/3750A<br />Breaking current 40 kA 50 kA (rms.)<br />Making current 100 kA 105 kA (peak)<br />Short ckt withstand 40 kA/sec 50 kA/sec<br />Closing time 52 milli sec 60 sec (III/IV) &amp; 30 sec (II)<br />Opening time 75 milli sec 35 sec (III/IV) &amp; 40 sec (II)<br />SF6 pressure 2.3 - 2.8 bar (2.2 alarm) at 20°C<br />Sliding contact Copper with silver of 10 microns<br />• What are the difference between DCCB and ACCB?<br />DCCB<br />Two poles seriesed for one side.<br />Breaker is adequately de-rated for use in dc circuits.<br />Only DINF, DIRS provided. DIT 5 will not work for dc.<br />In GFB, magnetic blowout coils used to increase the speed of rise of arc into the arc<br />chutes for effective quenching.<br />ACCB<br />The inherent current zero of sine wave helps arc quenching. For DCCB arc<br />quenching is difficult, since current zero is not existing naturally.<br />• What are the indications used in 415V and 6.6 kV breakers panel?<br />415 V Breaker 6.6 kV Breaker<br />Open Green Green<br />Close Red Red<br />Test White<br />Service Blue<br />Auto trip Yellow White<br />Spring charge Blue<br />Gas pressure Yellow<br />Voltage (RYB) Red<br />Question and answers Electrical Maintenance Unit<br />- 360 -<br />Question and answers Electrical Maintenance Unit<br />- 361 -<br />• What are the advantages and disadvantages of 415V English Electric breaker?<br />Advantages<br />1. Auto reclosing shutters.<br />2. Proven wiping contacts.<br />3. Sturdy rugged mechanism.<br />4. Reliable aux. Switches contacts.<br />5. Slow closing facility independent of closing spring.<br />Disadvantages<br />Bulky, more space, spring charge motor 5A. , Spring charging time 14 sec 4 sec at<br />timing, trip extends in test position also. No neutral bus bar link, we cannot finger<br />contact resistance since fixed on bus side.<br />• Type of closing spring is compression type.(415V)<br />• Why parallel operations of classIII buses are not permitted?<br />Fault on one side affects the other buses, switchgear fault level rating is exceeded.<br />• Why auto transfer is blocked for back up protection?<br />Because Backup protection operates normally for bus faults. All main protections are<br />generally operating for internal faults therefore there is no point in restoring the<br />power supply through auto transfer when there is a bus fault existing.<br />• What decides the control transformer VA rating?<br />Contactor coil VA rating.<br />• Can we use AC contactor in DC circuit?<br />Yes, but with adequate de-rating.<br />• Can we use ac coils in dc circuit?<br />Yes with economy resistor in series.<br />• Why shading rings provided in armature core of ac contactor. Why not for dc<br />contactor?<br />Because the force developed is not steady in ac therefore contacts will chatter but if<br />shading ring is used force developed becomes steady due to splitting of phases of<br />flux, therefore contacts becomes bounce free and humming sound reduces.<br />• Why copper contact are not used in contactor?<br />Because corrosion rate increases. Poor surface property , large closing force<br />required.<br />Question and answers Electrical Maintenance Unit<br />- 362 -<br />• Why pure silver is not used in contactor?<br />Affected by sulphur, mechanical or arcing damages (adv. Lower voltage drop)<br />Question and answers Electrical Maintenance Unit<br />- 363 -<br />• What are the types of contactors?<br />Type Contacts Rating Use<br />3 TB 41 2 NO + 2 NC 12 A 0 – 5.5 kW<br />3 TA 22 2 NO + 2 NC 30 A 7.5 – 11 kW<br />3 TA 13 2 NO + 2 NC 38 A 15 kW<br />3 TA 24 4 NO + 2 NC 70 A 18 – 38 kW<br />3 TA 16 4 NO + 2 NC 105 A 40 – 48 kW<br />3 TA 28 4 NO + 2 NC 170 A 55 – 80 kW<br />3 TA 28 3 NO + 3 NC 170 A 55 – 80 kW<br />3 TB 56 4 NO + 2 NC 400 A 93 kW for F/M supply PM.<br />3 TD 11 2 NO + 2 NC 12 A 0 – 1.5 kW for valve motors.<br />3 T I 22 2 NO + 2 NC 30 A 1.6 – 7.5 kW for valve motors.<br />• What material used for contactor?<br />Silver-nickel for <100A<br />Silver-cadmium oxide for large currents.<br />• Why pick up voltage is more than drop out voltage?<br />Initially air gap is more. Large force is required to overcome the high reluctance<br />initially. After closing air gap is reduced. Hence drop voltage is reduced.<br />• Can we file the pitted contacts of contactor?<br />No, use emery paper and etc.<br />• How the contact resistance can increase?<br />Humidity + salty air, dust, poor contact pressure<br />• Fusing current- the current at which the fuse element melts depends upon the<br />material, length and diameter.<br />• Fusing factor- fusing current /rated current (1.25 -1.75)<br />• Prospective fault current – first loop of fault current<br />• Cut off current - actual peak value of current reached due to interruption by fuse<br />blowing.<br />• What is rated current and short circuit current?<br />Rated current = VA / √3 * V Amps.<br />Short circuit current = VA * 100 / %Z * √3 * V Amps.<br />• What is the advantage of lower cut off current?<br />Question and answers Electrical Maintenance Unit<br />- 364 -<br />Less electromagnetic forces on contactor / CB’s<br />• How two fuses in-series are discriminated?<br />Total I2t of minor fuse should be less than pre-arcing I2t of major fuse.<br />Major fuse should be greater than 1.5 times the minor fuse.<br />Question and answers Electrical Maintenance Unit<br />- 365 -<br />• Why OLR time set high for belt driven fans?<br />Because acceleration time is high.<br />• What is the safety reason to keep the contactor / MCC remote from motor?<br />Contactor = sparking equipment. Not suiting for hazardous location.<br />• Where wound type CT used and where not used?<br />Used where low CT ratio req.<br />Not used where high short ckt. Current exists.<br />• What is advantage of cast resin CT’s?<br />Can withstand bursting forces under short ckt, protect damages against external<br />causes impervious to moisture.<br />• Why fuses with fusing factor more than 1.5 is not allowed in PVC cables?<br />Because PVC cables have low thermal capacity than paper cables. Full loading of<br />PVC only possible if it has close excess current protection (i.e.) Fusing Factor = 1.5<br />Question and answers Electrical Maintenance Unit<br />- 366 -<br />• What are the advantages of HRC fuses re-wirable fuses? What are the characteristics<br />of HRC fuse?<br />1. Consistent and stable characteristics for accuracy of discrimination. Capacity<br />to break at high and low current. It is inverse time characteristics, as the<br />current is high the time taken to break the circuit is less.<br />2. Arc quenching is reliable. Chemical action between quartz and arc gas gives<br />high resistance to the arc. Quartz does not produce more gas after observing<br />heat as its sand powder observes more heat of the arc.<br />3. Non deteriorating since it is sealed. No maintenance, Cheap and indication is<br />available.<br />Characteristics<br />1. I2t characteristics. This determines the energy that element can pass and to<br />determine the cut off characteristics.<br />105 Total I2t<br />104<br />103<br />102 Pre-arcing I2t<br />10<br />10 50 100 150 200<br />Fuse rating<br />2. Inverse time characteristics, which is useful for selection of the fuse for motor.<br />75<br />50<br />Current<br />20<br />10<br />0.2 0.5 0.7 0.9 1 sec<br />Time<br />Inverse time characteristics<br />Current<br />Fuse characteristics<br />Margin to avoid fuse operation<br />During starting<br />Minimum fusing current<br />Question and answers Electrical Maintenance Unit<br />- 367 -<br />Staring current<br />Running current<br />Time<br />Motor selection characteristics<br />Question and answers Electrical Maintenance Unit<br />- 368 -<br />• What is the advantage of CMM relay over normal Inv. O/C current relay?<br />Inv. O/C relay under protects at low current and over protects at high currents.<br />CMM: accounts for both +ve and –ve sequence currents i.e. Single phasing /<br />unbalance supply conditions and gives three times more weightage for the –ve phase<br />sequence current heating than + ve sequence current heating. i.e. Net rotor heating =<br />I1<br />2 + 3 I2<br />2.<br />Therefore CMM relay protection characteristic is closely matched to motor heating<br />characteristic. So it is better than thermal overload relay also.<br />• What is the purpose of anti-pumping relay?<br />When closing signal is continuously existing even after the closing of the breaker the<br />anti-pumping relay will be picked up and it will not allow the breaker to close back<br />in case of tripping of the breaker.<br />• Why breaker tripping is prohibited on very low pressure?<br />The efficiency with which the arc quenching is taking place in the breaker depends<br />on the air pressure. So if the air pressure is low effective arc quenching will not take<br />place which will result in damage of CB. Hence the tripping of the breaker at very<br />low air pressure is prevented.<br />• Why neutral breaker used in DG neutral grounding?<br />In case of high earth fault currents it is therefore normal practice to install a circuit<br />breaker in the neutral of the generator in order to reduce the total fault clearance<br />time.<br />• What are the protections used in Class III &amp; Class IV 415 V LV side?<br />Class III 415 V LV side<br />1. 51 (inverse over current)<br />2. 50 (instantaneous over current)<br />3. 27 (under voltage)<br />4. 51N (earth fault)<br />5. 64 (REF)<br />Class IV 415 V LV side<br />1. 51 RYB (inverse over current)<br />2. 51N (earth fault)<br />Question and answers Electrical Maintenance Unit<br />- 369 -<br />• What are the protections used in 415 V Class II side?<br />LV to UPS<br />1. CTZM<br />2. 50 (instantaneous over current)<br />3. short circuit<br />UPS to PMCC S &amp; T<br />1. 51 RYB (inverse over current)<br />2. 27 (under voltage)<br />3. 51N (earth fault)<br />From Class III to Class II tie<br />CTZM at Class III and 51, 51N at Class II.<br />• Define the followings.<br />Insulation level – it is the combination of rated voltage, the corresponding impulse<br />withstand voltage, which together characterize the insulation of the equipment as<br />regards its ability to withstand the electrical stresses.<br />Rated short circuit breaking current – it is the highest RMS value of short circuit<br />current which the circuit breaker is capable of breaking the circuit in safe.<br />Making current – it is the peak value of first loop of current of short circuit current<br />which the circuit breaker is capable of making at the rated voltage.<br />Rated making current = 2.5 times rated breaking current.<br />Short time rating – it is the RMS value of current that the circuit breaker can carry in<br />a fully closed position during a specified time.<br />Impulse withstand voltage – it is the amplitude of the standard voltage wave with the<br />insulation of equipment can withstand.<br />Power frequency withstand voltage – it is RMS value of alternating voltage wave of<br />power frequency (50 Hz) which the insulation of equipment should withstand.<br />• What is switchgear?<br />Equipment which is used for switching, controlling and protecting an electrical<br />circuit.<br />Question and answers Electrical Maintenance Unit<br />- 370 -<br />• Bus bar specifications of 6.6kV and 415 V.<br />415V<br />Aluminium 200 X 12mm<br />2 nos. per phase, 1 no. for neutral<br />Bus bar joints – silver plated to 5 micron thick (tightness 50 NM)<br />6.6kV<br />Copper<br />Silver coated joints<br />Insulation level – 27kV<br />Fault level estimated – 27kA Designed – 40kA<br />• What is the type of arc extinction in switchgears?<br />415V – resistance method (through arc splitter)<br />6.6kV – single puffer principle<br />• How Arc quenching is done?<br />When fault occurs depending on design element melts at one point and arc starts and<br />a transient current is super imposed on prospective current. When the sum of two is<br />zeroing the arc is quenching.<br />• What is lock out relay?<br />It is the relay to prevent the closing of circuit breaker after tripping (protection)<br />without attention of the operator.<br />• What is the significance of SF6 gas pressure in 6.6kV breakers?<br />Density gauge<br />Green – correct SF6 pressure (3 –3.5bar)<br />Yellow – pressure for breaking system fault current (refilling should be done)<br />Red – SF6 pressure less than 2 bar, which indicates leak in the system.<br />• What is the measure of atmospheric pressure and PSI?<br />1 Atmospheric pressure = 1.033 kg/cm2.<br />1 PSI = 0.07031 kg/cm2.<br />• What is the distribution of DC control supply in CL IV, III, II- 415V and 6.6 kV?<br />Closing coil and Trip coil 2 supply from one source.<br />Trip coil 1 supply from one source.<br />Protections supply from one source.<br />• Why 86.1 and 86.2 relays are used?<br />Question and answers Electrical Maintenance Unit<br />- 371 -<br />All electrical protection is wired to 86.1 and under voltage protection is wired to 86.2<br />relay for automatic restoration in EMTR.<br />Question and answers Electrical Maintenance Unit<br />- 372 -<br />• What is the difference between lockout of 6.6 kV and 415 V CB’s?<br />Voltage levels.<br />Manual / auto reset.<br />Lockout relays of 415 V breakers are all Electro-magnetic type. There is no<br />mechanical latch. But in 6.6 kV it is of mechanical latch type relays.<br />• If breaker contact resistance is more what action to be taken?<br />In 6.6 kV breakers 1250 A breaker contact resistance is <100 μς.<br />2000 A breaker contact resistance is <50 μς.<br />If contact resistance is found more than this value should be sent to the manufacturer<br />for repair.<br />• What are the interlocks between 415 V and 6.6 kV switch gear?<br />6.6 kV<br />Breaker closed cannot rack in or out.<br />Service lever cannot move while breaker closed.<br />Breaker cannot be closed in in-between position.<br />415 V<br />Breaker door cannot be opened when breaker is in service.<br />Breaker cannot be closed in in-between position.<br />Breaker closed cannot be rack in or out.<br />• Where are the provision of GR-A and GR-B tie possibilities?<br />Bus D1-D2 to E1-E2 (CL III 6.6 kV)<br />Bus X to Bus Y (CL III 415V)<br />Bus S to Bus T (CL II 415 V)<br />• What are the properties of SF6 gas?<br />This is inert gas. Odorless, non-toxic, colourless, stable, non-inflammable and<br />density is more hence high dielectric strength. The special property of this gas is<br />Electro negativity. This gas attracts electrons to form –ve ions and –ve ion are havier<br />than electrons and more slow in conduction, so that resistance in medium is increases<br />and get arc get extinguishes.<br />Question and answers Electrical Maintenance Unit<br />- 373 -<br />Question and answers Electrical Maintenance Unit<br />- 374 -<br />CABLES<br />• What are the purposes of cable trays?<br />a. Avoid sagging of cables<br />b. Give mechanical support<br />• What are the disadvantages of paper insulation?<br />a. Absorbs moisture.<br />b. Cable termination/sealing problems.<br />• What are the advantages of XLPE?<br />a. Easy routing at heights<br />b. Easy maintenance<br />c. Large current (90 ºC)<br />d. No sheath (no fatigue)<br />e. No paper tape wrapping technique<br />f. High dielectric strength<br />g. Very little deformation even at high temp,<br />h. More rated current, overload, short ckt capacity<br />i. Low tan delta and hence suits long routes<br />j. Very light<br />k. Good mechanical properties.<br />• What is the specified cable life?<br />50 years.<br />• How armours /sheaths grounded?<br />1 core cables -- sheaths/shields/armours grounded at one end only, other end<br />insulated to prevent the circulating current through sheaths.<br />3 core cables -- grounded at both ends but not including core balance CT’s, since<br />even small induced current causes 50N operation.<br />• Why armouring done?<br />For mechanical strength, protect against damage by impact of an object.<br />• How the required conductor size can be reduced by use of HRC fuses?<br />HRC fuse limits the peak amplitude of fault current. HRC fuse melts at prospective<br />current but not allows circuit to pass their high rupturing capacity that is it’s kA.<br />Question and answers Electrical Maintenance Unit<br />- 375 -<br />• What are the types of cables used in 6.6 kV and 415 V system voltages?<br />In 6.6 kV system AC (unearthed) grade having stranded aluminium conductors<br />cables are used. Their insulation’s are as follows.<br />1. XLPE – Crossed linked polyethylene insulation.<br />2. FRLS PVC – Fire retardant low smoke insulation with PVC inner and outer<br />insulation. In RB copper conductor stranded cables are used.<br />3. FS – fire survival insulation.<br />4. HR PVC – heat resistant insulation.<br />In 415 V system 1100 V grade copper or aluminium stranded cables are used. Their<br />insulations are HR PVC and FS type.<br />• At what temperature cables are rated?<br />Normally cables are rated for 40°C<br />Maximum temp in °C Short time temp in °C<br />PVC 70 160<br />HRPVC 85 160<br />Fire survival 90 250<br />Silicon rubber 90 250<br />XLPE cable 90 250<br />• Why 1.1 kV grade cables used for 415V?<br />To take care of the both earthed / unearthed systems.<br />• While carrying out cable joints, why should we ensure the continuity of 1) metallised<br />paper for PILC, 2) sheath and armour.<br />Continuity of metallised paper ensures less voltage gradient, hence preventing<br />puncture of insulation.<br />Continuity of sheath / armour ensures that grounding is maintained, so no over<br />voltage is induced, and easy to detect earthfault in cables.<br />• Why bimetallic washers provided in aluminium copper transition joints?<br />To avoid galvanic corrosion failure.<br />• What is the type cable used in radiation areas?<br />Mineral insulated (MI) cables.<br />Question and answers Electrical Maintenance Unit<br />- 376 -<br />Question and answers Electrical Maintenance Unit<br />- 377 -<br />EMTR AND AUTO TRANSFER<br />• What is meant by EMTR?<br />In case of normal supply failure to CL-III &amp; CL-II the loads will be fed by the<br />alternative/standby sources. This changeover of supply is called Emergency<br />Transfer.<br />• How EMTR is initiated?<br />EMTR is initiated on sensing the CL-III &amp; CL-II bus under voltage.<br />• What are the routes of EMTR?<br />CL-III bus under voltage. The DG’s will start. All the breakers connected to the<br />affected bus will trip. DG breaker will close on dead bus. Loads will be restored one<br />by one.<br />CL-II bus under voltage. The tie breaker of the affected bus will close.<br />• What does load-shedding mean?<br />When there is only one source to feed the two buses, the total loads can not be fed by<br />this single source. Hence some of the less important load will not be allowed to start<br />or it will be tripped if it is running.<br />• What does total load-shedding mean?<br />Even after the load shedding the is continue to deliver more than the rated power<br />sensed by overpower relay or running with under frequency sensed by the under<br />frequency relay the total load shedding will take place. In case of auxiliary<br />transformer is feeding the total load shedding will take place after 4 minutes.<br />• What does auto transfer mean?<br />If one of the sources is tripped on main protection its breaker will be tripped and the<br />tie-breaker will close. This transfer of supply from one source to other source is<br />called auto transfer. To restore the class IV whenever the UT or SUT is lost Auto<br />transfer is provided.<br />• Why auto transfer is prevented if the backup protection is operated?<br />The back up protection is supposed to operate in case of a bus fault. Hence the auto<br />transfer is prevented.<br />• What are the types Auto transfer scheme?<br />1. Fast transfer scheme to close the tie-breaker in less than 200 milli seconds. This<br />limits transient current and voltage dip in the bus and does quick acceleration of<br />the motors.<br />Question and answers Electrical Maintenance Unit<br />- 378 -<br />2. Slow transfer after 200 milli seconds.<br />Question and answers Electrical Maintenance Unit<br />- 379 -<br />• What are the conditions for Auto transfer scheme?<br />Backup protections are not allowed to initiate the Auto transfer. Because under<br />voltage may exist in the bus. Similarly protection in LV of the transformers are not<br />initiating the Auto transfer scheme. There are three metrics used in Auto transfer<br />scheme and 2/3 logic is adopted.<br />Conditions<br />1. Fault generator (86BG or 86A1).<br />2. Fault in UT.<br />3. Fault in GT.<br />4. Fault in 220 kV bus (SUT).<br />5. Fault in SUT (86M).<br />• What are the uses of EMTR scheme?<br />1. To restore class III when class IV supply fails.<br />2. To extend supply to class II when UPS fails.<br />3. In one DG condition to load restoration.<br />4. Sub sequent restoration of large motor loads.<br />• What are the sequences of motor load restoration in EMTR?<br />1. AHPPW –1001 - 4 Sec.<br />2. APWC – 1003 - 8 Sec.<br />3. APWC – 1004 - 12 Sec.<br />4. AHPPW –1002 - 16 Sec.<br />5. PPP – 1001 - 20 Sec.<br />6. MOD – 1002 - 24 Sec.<br />7. MOD – 1001 - 28 Sec.<br />8. Air Comp – 1002 - 32 Sec.<br />9. ABFP – 1006 - 36 Sec.<br />10. 7343 Exst Fan 1003 - 40 Sec.<br />11. ECCS PM 1001 - 44 Sec.<br />12. ECCS PM 1002 - 48 Sec.<br />• Why synchronizing scheme has been adopted? What are interlocks provided?<br />To check running and incoming buses, which are going to be interconnected are in<br />synchronism with each other.<br />Interlocks<br />1. Synch selector.<br />2. Only one breaker can operate at a time.<br />3. Master synch relay contact should available (bypass will bypass this synch<br />contact).<br />Bypass facility is provided to close the breaker on dead bus only.<br />Question and answers Electrical Maintenance Unit<br />- 380 -<br />• What are the settings provided in synchronizing scheme (SKE Relay)?<br />Voltage – V1- V2 = 10%<br />Frequency – t = 0.05 Sec (2.5 Hz)<br />% Slip – 0.45<br />Phase angle difference - 20°.<br />Question and answers Electrical Maintenance Unit<br />- 381 -<br />• What are the major losses in nuclear power generating system?<br />Condenser - 500 MW.<br />Moderator - 40 MW.<br />C/V and E/S system - 3.7 MW.<br />• Give the specification of DG and NGR of DG.<br />DG specification<br />Type HSPTL 12/653 synchronous generator.<br />KVA 2815<br />Volts 6600 V<br />Amps 246 A<br />Phase 3<br />Frequency 50 Hz<br />Insulation Class-F<br />PF 0.8<br />Duty S1<br />IC 01<br />IP 23<br />RPM 1000<br />Exciter 110 V, 3.2 A (Brush – less of permanent magnet, electronic<br />automatic voltage regulator)<br />Ambient temp 50°C<br />NGR specification<br />Resistance at 20°C 95.3Ω<br />Voltage 6.6 kV/√3<br />Insulation class 7.2 kV<br />Transient current 40 A / second.<br />Continuous rating 10 Amps.<br />• What is the operational requirement of DG’s?<br />1. Whenever class IV fails DG sets (2 + 1 standby) are started by EMTR and capable<br />of restoring class III loads within 30 Seconds. One example is given below.<br />Event Minutes Seconds Milli seconds Difference<br />Class IV fail 00 00 280 00:00:280<br />EMTR initiate 00 01 303 00:01:023<br />DG start (1, 2, 3) 00 01 336 00:00:033<br />Voltage, Speed reached 00 07 257 00:05:921<br />CB 351, 361, 370 Closed 00 07 491 00:00:234<br />Question and answers Electrical Maintenance Unit<br />- 382 -<br />Restoration will be done in 07 seconds and 491 milli seconds. After this to build up<br />power and frequency it takes about 1 minute 26 seconds and 852 milli seconds. Then<br />load restoration starts as per EMTR scheme.<br />2. DG’s are capable of paralleling with 6.6 kV class IV supplies.<br />3. DG’s are capable of paralleling with each other.<br />Question and answers Electrical Maintenance Unit<br />- 383 -<br />• What are the design criteria’s of DG’s?<br />1. 4000 starts and 4000 hrs run at full load for lifetime.<br />2. Operation at 45°C and high atmospheric condition.<br />3. Designed to run in earthquake and seismic condition.<br />4. Designed to start as per EMTR scheme and take load as per EMTR and load<br />shedding scheme in one-DG condition without drop in voltage or frequency 25%<br />and 5% respectively.<br />5. Designed for run in high speed and to build up voltage and speed within 10<br />seconds.<br />6. Designed to start and loading with external row water for cooling for 3 minutes.<br />7. Designed to start at normal and load condition temperature.<br />8. Designed to run at no load for 4 hrs in a 4 months with affecting the load and over<br />load.<br />9. Designed to supply power in one-DG condition.<br />10. Continuous supply is 2250 kW and can run at 2475 kW for 2 hrs in 24 hrs at 6.6<br />kV and 0.8 PF.<br />11. Designed to start and stop at 48 V DC supply and stop at 220 V DC in the case of<br />48 V DC is not available.<br />12. DG – 3 is physically separated for control and installed at adjacent unit. Because<br />in case of unit is not avail then for cooling water is available in other unit.<br />13. Monorail of 3 Ton is provided.<br />14. Provisions are made for filtered air and ventilation and combustion.<br />15. Co2 fire fighting system is provided for smoothening effect in case of fire.<br />16. Active process water from class III is provided.<br />• What are the auxiliaries required for DG?<br />1. Starting air system.<br />Components are compressor, air dryer, air receiver, solenoid valve for start<br />control, pneumatic starting air valve, air distributor and injection valve at each<br />cylinder.<br />This system operates at high pressure and also provided with soft start of 8-bar<br />pressure in testing of system periodically.<br />2. Lub oil system.<br />This is closed loop of having oil sump of capacity of 7 days at full load. This also<br />supplies oil to bearing lubrication, crankshaft, piston, and wiper.<br />This closed loop Circuit includes pre-lubricating circuit and normal lubricating<br />circuit. Pre- lubricating circuit is controlled by PLC, but at first commissioning<br />and overhauling pre-lub start immediately.<br />3. Water cooling system.<br />Question and answers Electrical Maintenance Unit<br />- 384 -<br />This is provided with closed loop fresh water for remove heat from lubricating oil<br />system, charge air cooler, engine components (cylinder lines, cylinder head etc).<br />Fresh water chemistry is controlled to avoid organic growth and corrosion. This<br />water is heated for normal operation to avoid thermal shock. This water is cooled<br />by active process water. One tank is provided to transfer fresh water to the<br />system. Before transferring fresh water chemical addition should be done.<br />4. Fuel oil system.<br />The engine driven fuel oil pump is flooded with fuel oil from the day tank by<br />gravity. Low-pressure fuel from fuel pump is supplied to individual injection<br />pump is injected to individual cylinders through injectors.<br />5. Combustion air and exhaust gas system.<br />The engine is supplied by compressed combustion air with the help of exhaust gas<br />driver Turbo – charger. Each bank cylinder is provided with a Turbo – charger.<br />Turbo – charger is provided with filters. The exhaust is passes through silencer.<br />6. Speed governing system.<br />Governer is hydraulic mechanical type. The governer is linked to the fuel racks.<br />The maximum work output of the UG – 8 governer is 8 lb – ft over the full 42°<br />travel. For full load 30°is sufficient and remaining for overloading.<br />Governer comprises<br />a. Speed droop setting.<br />b. Oil sight glass.<br />c. Load limiter<br />d. Compensation pointer and adjuster.<br />e. Local speed adjuster.<br />7. Two numbers of ventillation fans are provided in each DG building. One starts at<br />respective DG breaker close and other at >45°C. DG room is provided with 7<br />numbers of smoke detectors (ionized type) and 7 numbers of flame detectors<br />(photoelectric type).<br />8. Separate DG’s are having separate MCC for their auxiliaries power supply. For<br />DG – 1 auxiliaries MCC P1. For DG – 2 auxiliaries MCC Q1 and for DG – 3<br />MCC DG – 3 is provided.<br />9. Phase winding is provided with two RTD’s for hotspot measurement and bearing<br />provided with one RTD each.<br />10. LCP is provided one each for each DG.<br />• When DG’s start is not possible?<br />Question and answers Electrical Maintenance Unit<br />- 385 -<br />1. DG trip.<br />2. Turning gear engaged.<br />3. DG set stop push button pressed.<br />• What are stages of DG starting?<br />1. At start signal compressed air through solenoid valve passes over piston and<br />rotates the shaft.<br />2. At speed >60-rpm ignition starts.<br />3. Closing of excitation starts at >800-rpm.<br />4. At speed of > 900-rpm rated speed and rated voltage signal starts.<br />5. Closing of over speed is at >1150-rpm.<br />Question and answers Electrical Maintenance Unit<br />- 386 -<br />• What are the protections provided for DG?<br />1. Differential protection (87).<br />2. Over speed of engine.<br />3. Reverse power protection (in LOCA condition time delay).<br />4. Low lubricating oil pressure.<br />5. Cooling water temperature high.<br />6. 6.6 kV switchgear protections.<br />7. Excitation failure.<br />8. Emergency stops push button.<br />In LOCA condition 4 – 8 protections are not permitted to operate.<br />Question and answers Electrical Maintenance Unit<br />- 387 -<br />Electronics<br />• What is Diode?<br />Diode is a two-layer semiconductor device, conducts only positive cycles when<br />applied to anode.<br />• What is Thyristor?<br />Thyristor is a four or more layer semiconductor device &amp; having 3 or more<br />junctions. It is also called Silicon Controlled Rectifiers (SCR). A healthy SCR must<br />block in both the directions at least 1MΩ resistance, a fused SCR will conducts in<br />both directions.<br />• What is material used in making semiconductor?<br />Silicon &amp; Germanium are the raw materials used for making semiconductor.<br />Semiconductors are located between conductors &amp; insulators in the resistivity<br />spectrum &amp; allow current to flow only under certain conditions.<br />• What is material used in making non-linear resistor &amp; purpose of it in field discharge<br />resistor?<br />Silicon carbide materials used for making non-linear resistor. The purpose of this<br />resistor is to avoid surge voltage when field breaker opens. These resistors are<br />connected in parallel to the main field winding (Rotor).<br />• How over voltage is produced in Field breaker?<br />Over voltages appear if synchronous generators and motors fall out of step inducing<br />an AC voltage in the field system. Depending on the type of construction of the<br />machine and the slip this voltage can become un-permissibly high, for this purpose<br />over voltage protectors are provided in the field breaker cubicle.<br />• What is firing angle?<br />The angle in the AC cycle at which the thyristor starts conducting at the application<br />of positive voltage to gate is known as the firing angle (α)<br />• What is Inverter operation?<br />When firing angle a = 90º the positive &amp; negative voltages areas are equal. With a<br />higher than 90º the negative areas are greater so the total voltage becomes negative.<br />This condition is termed as 'Inverter Operation".<br />• What is the purpose of RC network across thyristor?<br />RC network across each thyristor protects against Hole Storage Effect.<br />• Why reactors are provided in Thyristor bridges?<br />Question and answers Electrical Maintenance Unit<br />- 388 -<br />Reactors are provided to limit the rate of rise of current (di/dt) in the device, thereby<br />avoiding possible damage to the device. They also effect a proper sharing of load<br />among thyristor bridges when connected in parallel.<br />Question and answers Electrical Maintenance Unit<br />- 389 -<br />• What is purpose of connecting thyristor bridges in parallel?<br />Thyristor bridges are connected in parallel to improve current rating.<br />• What is purpose of Load angle limiter?<br />Load angle limiter, it either limits the angle between grid load center and the rotor<br />axis or generator terminal &amp; rotor axis.<br />• What is purpose of Rotor angle limiter?<br />It limits generator voltage &amp; rotor voltage.<br />• What is purpose of Rotor current limiter?<br />It limits overloading of rotor.<br />• What is purpose of Stator current limiter?<br />It limits the stator current.<br />• What is purpose of Slip stabilization?<br />It avoids oscillation of the AC machine (Rotor oscillations).<br />• What is purpose of reactive power (VAR)?<br />It is an energy required to built up magnetic field to drive the power.<br />• What is the advantage of Static Excitation?<br />Fast response time, high reliability, interchangeability of parts during operation, less<br />wear &amp; tear due to static devices &amp; less maintenance.<br />• What is the advantage of field forcing in the rotor?<br />Field forcing acts for 10 seconds to maintain the generator terminal voltage during<br />fault condition so as to operated the protection relays.<br />• When the negative sequence reactance arises?<br />Negative sequence arises whenever there is any unbalance present in the system.<br />Their effect is to set up a field rotating in opposite direction to the main field.<br />• When the Zero sequence reactance arises?<br />If a machine is operating with an earthed neutral, a system earth fault will give rise to<br />zero sequence current in the machine.<br />Question and answers Electrical Maintenance Unit<br />- 390 -<br />Basic Electricity<br />• What is Current?<br />The flow of electrons in a circuit is called current, it is measured in Amperes (I).<br />• What is Voltage?<br />Voltage is the difference in potential (charge) between two points or voltage is the<br />amount of driving force or pressure applied to a circuit, it is measured in Volts (V).<br />• What is Resistance?<br />The resistance of a circuit is the circuit's opposition to the movement of electrons. A<br />resistor restricts or limits the amount of current flowing in a electrical circuit, it is<br />measured in Ohm (Ω).<br />Series Resistor: When resistors are connected in series they have one point in<br />common. The total resistance is equal to the sum of the individual resistors.<br />R tot = R1 + R2 + R3<br />The current in a series circuit is the same in each component of the circuit because<br />the current must flow through each resistor in series to get to the next resistor.<br />I tot = I1 = I2 = I3<br />The applied voltage divides across each component in a circuit in proportion to the<br />resistance of the component. V tot = V1 + V2 + V3<br />Parallel Resistor: When resistors are connected in parallel, they have two points in<br />common. The total resistance of parallel resistors is equal to the reciprocal of the<br />sum of the reciprocal of the individual resistors. R tot of a parallel circuit is called the<br />equivalent resistance,<br />R eq = 1<br />1/R<br />1<br />+ 1/R<br />2<br />+1/R<br />3<br />Question and answers Electrical Maintenance Unit<br />- 391 -<br />• Why color-coding is necessary for resistor?<br />A wide variety of resistors are physically large enough to have their resistance value<br />printed on them. However, carbon composition resistors are too small for this<br />method of identification so a color coding system is used. Four bands are printed on<br />one end of the resistor and are read from the band closest to the end of the resistor<br />toward the center. Each color represents a numerical value as indicated below.<br />0- Black 3- Orange 6- Blue 9- White ±5% - Gold } Tolerance<br />1- Brown 4- Yellow 7- Violet 0.1- Gold ±10% - Silver }<br />2- Red 5- Green 8- Grey 0.01- Silver<br />Suppose the color bands of a resistor are yellow, violet, red and gold. The resistance<br />value is determined as follows:<br />4 7 00 = 4700Ω<br />Yellow = 4<br />Violet = 7<br />Red = 2 (two zeros)<br />Gold = ±5%<br />4700+5% = 4935} The actual resistance should be between 4467 and 4935 ohms.<br />4700-5% = 4465}<br />Occasionally a fifth band is used to indicate the failure rate of the resistor:<br />Yellow 0.001% per 1000 hours<br />Orange 0.01% per 1000 hours<br />Red 0.1% per 1000 hours<br />Brown 1.0% per 1000 hours<br />• What does Ohm's Law states?<br />In a closed electrical circuit, current is directly proportional to voltage and inversely<br />proportional to resistance at constant temperature.<br />I= V/R. Where I= Current, V= Voltage &amp; R= Resistance<br />• What does Kirchhoff's voltage law states?<br />Kirchhoff's voltage law states that " the algebraic sum of potential rises and drops<br />around a closed loop is zero." Σ􀀴V= 0<br />• What does Kirchhoff's current law states?<br />Kirchhoff's current law states that " the algebraic sum of current entering and leaving<br />a node is zero." (A node is a junction of two or more branches.)<br />• What is a Capacitor?<br />When two conductors are placed side by side, separated by a nonconductive<br />material, and connected across a battery, free electrons drift in the direction of the<br />Question and answers Electrical Maintenance Unit<br />- 392 -<br />driving voltage is called capacitor. Its unit is farad, normally in micro farad (μf) or<br />Pico farad (pf).<br />Question and answers Electrical Maintenance Unit<br />- 393 -<br />• What is Inductor?<br />Inductors use the ability of electrical current to create a magnetic field. If a voltage is<br />applied to a coil of wire, the current flowing in the coil will cause a magnetic field to<br />develop. The more times the wire is coiled and the more current there is in the coil,<br />the greater the strength of the magnetic field. Its unit is Henry, normally in milli<br />Henry (mH).<br />• What is Inductance?<br />The property of a coil that opposes a change in the current flow is called inductance.<br />The inductance of a coil depends on four factors:<br />1) The number of turns (windings) in the coil. Inductance is proportional to the<br />square of the number of turns in the coil.<br />2) The diameter of the coil. The larger the diameter of the coil, the higher the<br />inductance.<br />3) The permeability (ability to become magnetized) of the core material.<br />4) The length of the coil. The shorter the coil, the higher the inductance.<br />• What is Power?<br />Power is a rate of doing work, or works done per unit in time. The unit for measuring<br />power is the Watt (W). Power in watts is equal to the product of the applied voltage<br />and the current flowing. Stated algebraically, P = I E<br />• What is Alternating Current?<br />Continually changing amount and direction of the current and voltage is called<br />alternating current (AC). The components of an ac circuit causes a time period to be<br />introduced between current and voltage; that is, current and voltage are out of phase.<br />Question and answers Electrical Maintenance Unit<br />- 394 -<br />Few definitions and symbols used in alternating current<br />1) Amplitude or peak value: The maximum value reached by a waveform.<br />2) Capacitive reactance (XC): measured in ohms, is the opposition to a change in<br />current flow provided by a capacitor. Capacitive reactance causes current to lead<br />voltage by 90°: Xc = 1/2p f C.<br />3) Cycle: The portion of a waveform contained in one period of time.<br />4) Effective value: The value of voltage that occurs at 45° (0.7071 times the<br />maximum value).<br />5) Frequency: The number of cycles per second is called frequency and measured in<br />Hertz (Hz).<br />6) Impedance (Z): The opposition to current flow in an ac circuit. It is a combination<br />of resistance, Inductance and capacitance.<br />7) Inductive Reactance (XL): Inductive reactance, measured in ohms, is the<br />opposition to a change in current flow produced by a coil of wire. Inductive<br />reactance causes current to lag voltage by 90° : XL = 1/2π f L.<br />8) Instantaneous value: The magnitude of a waveform at any instant of time.<br />9) Period (T): The time interval between successive repetitions of a periodic<br />waveform.<br />10) Periodic waveform: A waveform that continually repeats itself after the same<br />time interval.<br />11) Resistance: the opposition of a circuit to the movement of electrons. Resistance<br />in an ac circuit acts the same as resistance in dc circuit.<br />Question and answers Electrical Maintenance Unit<br />- 395 -<br />Measuring Instruments<br />• What is Voltmeter?<br />It is an instrument to measure voltage. It is always connected in parallel to the power<br />supply.<br />• What is Ammeter?<br />It is an instrument to measure the current flowing in a circuit. It is always connected<br />in series with the load.<br />• What is Megger?<br />The megohmmeter, commonly called a megger for short, is used to measure very<br />high resistance values. It is primarily used to test the insulation of conductors. To<br />measure high resistance values, a high voltage is applied, either by the use of a handcranked<br />generator or electronic power supply.<br />• What is Clamp-On Ammeter?<br />Clamp-on ammeter is used to check the current in a circuit, without being physically<br />connected in a circuit. They are convenient to use in the field since the circuit does<br />not have to be opened to take a current reading.<br />• What is Infrared or Thermal scanner?<br />Infrared or thermal scanners are used to measure temperature without contact with<br />the equipment. They produce an image of the component showing temperature<br />variations, this is effective in spotting worn or loose connections and components in<br />industrial circuits.<br />• What is Phase sequence indicator?<br />Phase sequence indicator used to indicate the 3-phase direction- comes in two styles;<br />Lights and meters. In the lighted variety, a sequence of light goes on for the phase<br />sequence being read, while the meter indicates which phase direction it is reading.<br />• What is Rotation tester?<br />This device is used during the installation of a motor to determine the direction of the<br />motor once it is installed. The shaft is mechanically rotated in the desired direction<br />and the meter indicates if that is the direction in which the motor will rotate.<br />Question and answers Electrical Maintenance Unit<br />- 396 -<br />DC Machines<br />Conversion of one form of energy into another enables us to use natural power<br />sources as well as manufactured power sources to produce our electrical power<br />supply. Although electricity can be produced by friction, pressure, heat, light,<br />chemical action and magnetism, the most common method used by large power<br />producers is magnetism.<br />• What is Electric Generator?<br />Electric generators are called a dynamo that converts mechanical energy into<br />electrical energy. A dynamo consists of two basic parts- the stationary part and the<br />rotating part.<br />• How electromotive force is created in a generator?<br />When a conductor cuts the magnetic lines of forces, an Electro motive force (emf) is<br />generated.<br />The magnitude of the generated voltage is directly proportional to the rate of change<br />at which a conductor cuts the magnetic lines of force.<br />• What is DC motor?<br />An electric motor converts electrical energy in to mechanical energy.<br />• How many types of DC motors are there?<br />DC shunt motor: shunt motor speed varies slightly from no load to full load.<br />DC series motor: series motor speed varies greatly as load changes.<br />DC compound motor: the compound motor contains both a shunt field and a series<br />field and therefore has characteristics between the shunt and the series motors. This<br />motor has the good starting torque characteristics provided by the series field, while<br />the shunt field provides for a relatively constant speed.<br />Question and answers Electrical Maintenance Unit<br />- 397 -<br />Maintenance &amp; Troubleshooting<br />• Troubleshooting is a field if repair work that usually tells how well the student<br />has learned the lessons. The principles involved in control functions, components<br />and circuit analysis, along with the basic laws of electricity.<br />• Your best tool when troubleshooting is your ability of think. Don't jump to<br />conclusions. Have confidence in your ability. Learn how the equipment in your<br />area is supposed to operate both electrically and mechanically.<br />• Observe all plant rules and regulations. Electricity can be dangerous. In addition<br />to the hazards of electrical shock and electrocution, burns from an electrical flash<br />can be devastating. Be careful when opening the circuit. The inductive kick that<br />can occur when a circuit opens produces a voltage that is many times the voltage<br />applied to the system.<br />• No matter how complex or expensive an electrical control system is, the<br />components of the system begin to deteriorate as soon as they are installed and<br />failure of some components in the system will ultimately result.<br />• Blown fuses, overload contacts, open contacts, short circuits, burned out coils and<br />grounds are responsible for most electrical circuit failures.<br />• Troubleshooting can be generalized in 3 steps:<br />1) Determine the symptoms; that is, find out how it acts. (When equipment is<br />operating properly, you should find out how it is supposed to function.)<br />2) Decide by logical reasoning what might be wrong. (Try to isolate the problem<br />to a section of the control.)<br />3) Determine what has to be done to correct the problem.<br />• If we are troubleshooting an existing circuit, one that has been in service and<br />operated properly, we can eliminate the possibility of fault installations or design.<br />• The first step- determine the symptoms- can best be accomplished by working<br />with the machine operator and following the machine through its sequence to the<br />point of failure.<br />• Remember that no matter how complex, control circuit are made up of only two<br />things. Contacts that open and close a circuit and coils that operate the contacts,<br />keeping in mind the control voltage.<br />• Probably the single most important rule in trouble shooting is to remember to<br />change only one thing at a time.<br />• Remember the operator knows the machine operation and can be an asset to you<br />in your troubleshooting. Question the operator but don't challenge his operating<br />ability.<br />Question and answers Electrical Maintenance Unit<br />- 398 -<br />• Anyone attempting to troubleshoot without a drawing and a meter is wasting the<br />time.<br />• Instead of random checking the circuit; start from the source to the machine or<br />from the machine to the source.<br />• Finally take time to think.<br />Question and answers Electrical Maintenance Unit<br />- 399 -<br />Radiation Protection<br />Fission reaction<br />92U235<br />0n1<br />54Xe144 + 38Sr90 + neutron + radiation + Energy<br />Tritium formation<br />1H2<br />0n1<br />1H3<br />1 Seivert = 100 Rem<br />Annual Dose Limit (ADL) = 20 mSev or 2 Rem for Employees.<br />Annual Dose Limit (ADL) = 1 5 mSev or 1.5 Rem for Contractor.<br />Annual Dose Limit (ADL) = 1 mSev or 100 mRem for Public.<br />5 Years = 100 mSev or 10 Rem<br />DAC (Derived Air concentration)<br />>10 DAC use tritium bottles<br />10-15 DAC use airline<br />>50 DAC use ventilated plastic suite (VP suite)<br />1 DAC for 1 hour = 0.01 mSev or 1 mRem.<br />• Why no entry for Moderator room &amp; Pump room during operating condition?<br />Due to the presence of N16 &amp; O17, which are high gamma emitter, their field is<br />around 7 mev.<br />• What are the gases discharged to the stack?<br />Argon-41, Tritium, fission products, noble gases &amp; Iodine particulates.<br />Question and answers Electrical Maintenance Unit<br />- 400 -<br />• What are the emergencies provided in the plant?<br />Plant Emergency: Excessive release of radioactive material or high radiation fields in<br />a section of the plant<br />Site Emergency: Uncontrolled release of radioactive material or high radiation fields<br />with in the site boundary<br />Off- Site Emergency: High release of radioactive material from the plant resulting in<br />significantly increased radiation fields and/or contamination levels extending to<br />areas outside the site<br />Emergency Planning Zones (EPZ): Emergency planning zone, defined around the<br />plant up to 16 km, provides a basic geographic frame work for decision making<br />on implementing measures as part of a graded response in the event of an<br />emergency. The area around the Kaiga generating station is divided into the<br />following Zones up to 16 km radius.<br />Exclusion Zone: The exclusion Zone extends up to a distance of 1.6 km around the<br />central plant zone of 0.7 km where no public habitation is permitted. This zone is<br />physically isolated from out side areas by plant fencing and is under the control of<br />Kaiga Generating Station.<br />Sterilised Zone: Sterilised zone is an area where no new growth of population is<br />permitted. Natural growth is however allowed in this Zone. This are extends up to<br />a radius of 5 km from the central plant Zone. This Zone is defined to restrict the<br />population to an easily transportable number in case of an emergency.<br />Primary Zone: The primary Zone extends up to 8 km from central part Zone where<br />protective measures like evacuation and sheltering are required against possible<br />plume exposures during an Emergency.<br />Secondary Zone: The secondary Zone extends up to 16 km from central plant Zone<br />protective measures like sheltering control on food stuff are required against<br />possible exposure from ingestion of radioactivity.<br />Question and answers Electrical Maintenance Unit<br />- 401 -<br />CLASSIFICATION OF EMERGENCIES<br />Emergencies are classified on the basis of the nature and severity of the incident. The<br />effect of the emergency may be restricted either to a small area of the plant or a few<br />individuals or it may pose damage to the installation staff. Emergencies of more<br />severe nature could result in unacceptably enhanced release of radioactive materials<br />or toxic/noxious substance from the plant of resulting in hazard in the surrounding<br />public domain. Accordingly the emergencies are classified into:<br />1. Plant emergency<br />2. Site emergency<br />3. Off-site emergency<br />Plant Emergency<br />This type of emergency is classified in to<br />a) Personal emergency<br />b) Emergency Alert<br />c) plant emergency<br />Personal Emergency: This involves accidents or incidents in any of the plant areas,<br />which call for emergency treatment of personal. The situation may result from<br />high radiation exposure or significant contamination or abnormal intake of<br />radioactivity by personal. The examples of personal emergencies are listed in<br />Annexure-I.<br />Emergency Alert/Emergency Standby: This involves abnormal conditions, which<br />have a potential to proliferate in to a more serious situation but still provide time<br />for pre-cautionary and constructive steps to prevent an emergency situation or<br />migrate its consequences. The examples of emergency Alert are listed in<br />Annexure-II.<br />Plant emergency This involves excessive release of radioactive materials or high<br />radiation fields in a section of the plant requiring operator action and/or automatic<br />operation of the safety system. Although positive isolation or restriction on<br />occupancy of the affected areas might be enforced, evacuation of personal might<br />be required if it is suspected that the doses to personal or likely to exceed the<br />intervention levels. The examples of plant emergency conditions or listed in<br />Annexure-III.<br />Site Emergency<br />This class of emergency arises due to situation, which seriously affect plant<br />operation involving high radiation fields in accessible areas and release of<br />radioactive materials extending beyond the plant up to the site environment. The<br />protective measures such as incorporation of stable Iodine, sheltering and evacuation<br />of personal from plant areas other than control room to areas designated to be<br />habitable under the site emergency conditions and evacuation of non-essential<br />persons from the site may be considered. The examples of site-emergency condition<br />are listed in Annexure –IV.<br />Question and answers Electrical Maintenance Unit<br />- 402 -<br />Off-site Emergency<br />An Off site emergency situation results when the release of radioactive materials<br />from the plant is of a magnitude necessitating protective action to be taken for<br />members of the public in the neighborhood of the plant.<br />Question and answers Electrical Maintenance Unit<br />- 403 -<br />EMERGENCY DECLARATION AND NOTIFICATIONS:<br />Declaration of Emergencies shall be made by the Duty SCE / PED based on the<br />information from the plant or as per the advice from Kaiga emergency Committee<br />(KGEC).<br />Declaration of Emergency: Siren will be sounded as described below for declaring<br />emergency. Following the Siren, there should be an announcement.<br />Siren: Short intermittent siren 5 seconds on, 5 seconds off for a period of two<br />minutes.<br />Emergency Announcement:<br />The announcement shall be made as follows;<br />"ATTENTION ALL PERSONNEL - THERE IS PLANT EMERGENCY"<br />THE INCIDENT AREA IS …………………………………….<br />THE ASSEMBLY AREA IS ……………………………………<br />THE EMERGENCY CONTROL CENTRE IS……………...…..<br />PERSONS PRESENT AT …………… SHOULD AVOID GOING TO ………...<br />This announcement shall be repeated thrice in English, Hindi and Kannada.<br />Evacuation: Evacuation if necessary will be made by announcement on Public<br />Address (PA) system.<br />Termination of Emergency: A continuous Siren is sounded for 2 minutes. Following<br />the emergency Siren, there shall be an announcement in English, Hindi and Kannada<br />on public address system terminating the emergency.<br />Notification Codes:<br />The messages for notification of start/termination of on site and off-site emergencies<br />are indicated as follows. These should be disseminated to various agencies. The<br />codes for notification of commencement or termination of various types of<br />emergencies are:<br />a) External radiation exposure (mSv) DAC-hr (HTO) DAC-hr(I-131)<br />DAC-hr (I-131) ------(≤ 1)<br />(For meeting iodine thyroid dose limit of 50 mSv)<br />The explanatory notes for these guidelines are given in Annexure-IX.<br />Question and answers Electrical Maintenance Unit<br />- 404 -<br />Countermeasures during a radiation emergency: Following countermeasures have<br />been identified for control of exposures during a radiological emergency within the<br />plant site areas and in the public domain.<br />1. Sheltering<br />2. Administration of Stable Iodine<br />3. Evacuation.<br />4. Relocation.<br />5. Control of Access.<br />6. Control of Food and Water<br />7. Decontamination of Affected Areas and Buildings.<br />DOMAIN:<br />Domain 1 = 0.1 mSv/hr<br />Domain 2 = 0.01 mSv/hr<br />Domain 3 = less than 0.01 mSv/hr<br />Stochastic and Deterministic effects.<br />Stochastic effects: Stochastic effects are those for which the probability of an effect<br />occurring, rather than its severity, is regarded as a function of dose, without<br />threshold. Example: Cancer.<br />Deterministic effects: Deterministic effects are those for which the severity of the<br />effect varies with the dose, and for which a threshold may, therefore, occur.<br />Examples Cataract, permanent or temporary sterility.<br />Practices: Any human activity, which increases the overall exposure to radiation, is<br />a "Practice" such as operation of nuclear power stations.<br />Intervention: Any human action intended to reduce or avert exposures to sources<br />which are not part of controlled practices or which are out of control as a<br />consequence of an accident is "Intervention".<br />Objectives of Radiation Protection: Prevent deterministic effects and to limit the<br />stochastic effects to levels deemed to be acceptable.<br />Question and answers Electrical Maintenance Unit<br />- 405 -<br />Principles of Radiation Protection:<br />Practices:<br />a. Justification: No practice shall be adopted unless its introduction produces a<br />sufficient benefit to the exposed individual or to the society to offset the<br />radiation harm that it might cause.<br />b. Optimisation: All exposures shall be kept As Low As Reasonably Achievable<br />(ALARA) economic and social factors being taken into consideration.<br />c. Dose limitations: Individual exposures are limited by dose limits since the<br />dose above the dose limits are unacceptable<br />Intervention: The general principles of radiological protection for intervention are:<br />a. The reduction in dose should be sufficient to justify the harm and the costs to<br />the individual and the society due to the intervention.<br />b. The benefit of the reduction in dose less the cost of intervention should be As<br />Large As Reasonably Achievable.<br />c. Dose limits do not apply in case of intervention. However there will be some<br />projected dose levels above which intervention will be justified because of<br />serious deterministic effects.<br />Dose limits: Occupational Workers<br />a. For stochastic effects: The dose limit for uniform irradiation of the whole body<br />shall be 20 mSv (2 Rem) averaged over 5 years (January 1,1999 to December<br />31,2003) and shall not exceed 30 mSv (3 Rem) in a single year.<br />b. The average whole body dose for the occupational workers in the station<br />should normally not exceed 5 mSv (500 Rem).<br />c. For deterministic effects, the dose limit shall be 500 mSv (50 Rem) in a year<br />to Bone surface, Skin and for the lens of the eye, for which the limit shall be<br />150 mSv (15 Rem) in a year.<br />The whole body exposure level should Remain less 10 mSv (1 Rem) in any month<br />and 15 mSv (1.5 Rem) in any calendar quarter.<br />In case of intakes of radioactive material into the body, the total amount of activity<br />taken into the body in a calendar year shall not exceed one ALI (Annual Limit on<br />Intake).<br />Incase of exposure resulting from both external radiation and intake of radionuclides<br />in the body it shall be ensured that the sum of effective dose resulting from all such<br />exposures does not exceed the annual dose limits.<br />Whole body dose Ii -- + Σ -- < 1<br />0.02 Sv (ALI) I<br />Where Ii is the intake of the i th radio-nuclide and (ALI) i the ALI value for the i th<br />radio-nuclide.<br />Question and answers Electrical Maintenance Unit<br />- 406 -<br />Planned Exposure: Situation may occur in-frequently during normal operations when<br />it may be necessary to permit a few workers to receive dose in excess of the annual<br />whole body dose limit. In such circumstances, Station director may permit exposure<br />such that dose does not exceed 30 mSv in a single year and 20 mSv averaged over 5<br />years.<br />Question and answers Electrical Maintenance Unit<br />- 407 -<br />External Exposure Control: Any external whole body exposure that exceeds 5 mSv<br />(0.5 Rem) in any month is referred to as significant dose.<br />Internal Exposure Control<br />a. Proper ventilation of work areas and use of the recommended protective<br />equipment would avoid intake of radionuclides in the body.<br />b. In any case of actual or suspected high intakes HPU should be contacted for<br />advice and appropriate action.<br />KGS-Operating Manual on Radiation Protection Procedures<br />For assessment of internal exposure due to tritium, bioassay of urine will be taken as<br />the standard reference. For assessment of internal exposure by radionuclides other<br />than tritium bioassay and/ or whole body counting whichever is applicable will be<br />taken as standard reference. For control of intake of tritium the following procedures<br />shall be l Exposure Control followed:<br />Permissible Contamination levels<br />Air borne Contamination the levels of air borne contamination in working areas at<br />the station should be maintained below the Derived Air Concentration (DAC) values<br />DAC (Bq/m3) = ALI Bq/2400 m3<br />Investigation of Doses<br />Investigation levels Whole body dose: Committee (SDIC) shall investigate these<br />exposures<br />Dose Reference Levels for Investigation<br />Tissue/Organ Investigation Levels mSv (Rem)<br />Monthly Quarterly Yearly<br />Whole body 10 (1) 15 (1.5) 20 (2)<br />Skin 100 (10) 300 (30) 500 (50)<br />Lens of Eyes 30 (3) 80 (8) 150 (15)<br />Question and answers Electrical Maintenance Unit<br />- 408 -<br />The functions of SDIC:<br />a. To investigate fully the causes of the doses above the investigation levels and<br />to prepare a factual report.<br />b. To suggest Remedial measures to prevent recurrence of such doses.<br />c. To suggest further action in respect of work to be allocated to the exposed<br />person.<br />d. To recommend Remedial measure and medical follow up wherever necessary.<br />Exposure exceeding any of the limits stipulated below shall be regarded as<br />potentially serious:<br />Whole body dose: 100 mSv (10 Rem)<br />Exposure to Eye Lens: 300 mSv (30 Rem)<br />Such cases shall be referred to Head, Medical Group, BARC and Chairman,<br />SARCOP immediately<br />Head, Medical Group, BARC shall initiate appropriate medical investigation<br />Medical report shall be submitted to Chairman, SARCOP, within a week. Chairman,<br />SARCOP shall constitute a special committee for investigation of such exposures.<br />Tritium half life: Radiological =12.3 years, Biological = 7 days, tritium effective =<br />(TR *TB) / (TR +TB)<br />(12.3*365*7) / (12.3+365+7) = 7 Days<br />REVERSE SQUARE LAW: Dose at a rate form the point of source is inversely<br />proportional to the square of the distance. I is inversely proportional to L / d square<br />Technical specification Values: Fission products<br />Noble gases 14.8 TBq / day, Tritium = 13TBq/day, Ar 41 = 2.04 TBq / day,<br />I-131 = 185 MBq/day, Particulate = 1480 MBq / day<br />Liquids: Tritium = 1.295 TBq/day<br />RADIOACTIVE TRANSPORT INDEX: 1 meter from the source shield. Declaration<br />of radioactive material = 70 kiloBq / kg<br />Question and answers Electrical Maintenance Unit<br />- 409 -<br />Maintenance Performance Planning<br />1. Essential sequence of maintenance Jobs execution.<br />Identify maintenance jobs, asses safety, radiological precautions, plan the work,<br />do the pre-job briefing, take the safety and ALARA measures, carry out the<br />maintenance, test and normalise equipment or system, update records and history<br />cards, review maintenance performance and devise future strategy, achieve<br />excellence in maintenance through dedicated team work.<br />2. Maintenance performance indicator based on equipment &amp; work control<br />Maintenance performance indicator (MPI) is the measure of performance of each<br />aspects of maintenance. These are established as convenient measures to evaluate<br />current performance levels against standard as well as an index to compare with<br />past performance.<br />MPI base on equipment performance (EMPI)<br />a) Equivalent availability % (should be as high as possible)<br />Equipment operating time x 100<br />Equipment operating time + down time<br />b) Mean time between failures (MBTF) should be as high as possible<br />Number of operating hours<br />Number of failures/breakdowns<br />c) Meantime to repair (MTTR) as low as possible<br />Sum of repair time<br />Number of breakdowns<br />d) Number of plant outage caused due to equipment failure. (Objective should<br />be zero)<br />e) Number of respective failures during reporting period. (as low as possible)<br />f) Number of breakdowns during reporting period (As low as possible)<br />3. Maintenance Performance indicator based on work control<br />a. Work control indicator (WCI) should be near to unity<br />No. of DR received from control room per month<br />No. of PM jobs planned<br />4. Maintenance performance indicator based on maintenance man hours<br />1) Man hours spent on breakdown maintenance<br />2) Man hours spent on PM including implementation of ECN/FCN’s etc.<br />3) % man hours spent on breakdown maintenance<br />Man hours spent on = Breakdown maintenance 100<br />Total maintenance man hour available<br />4) % of man hours spent on PM<br />= Man hours spent on PM x 100<br />Question and answers Electrical Maintenance Unit<br />- 410 -<br />Total maintenance man hour available<br />Question and answers Electrical Maintenance Unit<br />- 411 -<br />• What is FME explaining with the examples?<br />Foreign material is defined as material that is not part of a system or component as<br />designed. This includes dirt, debris, broken or missing parts, slag, tools rags,<br />liquids/chemicals, lapping compounds, grinding particles and any other item that<br />would affect the intended operation of a system or component<br />All personnel shall assume responsibility for preventing the introduction of foreign<br />material into systems. This will minimize damage or harmful effects. Such as<br />corrosion, fuel damage, component malfunction, or failure, changes in chemistry.<br />Reduced heat transfer, increased radiation levels, changes in system flow<br />characteristic and improper contact operation.<br />Specific actions includes the following<br />Work packages will be planned using field walk downs to determine specific FME<br />recommendations<br />If temporary dams are installed which will not be readily visible upon system<br />closure, verification of removal shall be included in the checklist.<br />• What is the importance of communications?<br />Effective, open communication is essential for safe and efficient performance of<br />plant maintenance. Expressing concerns describing assignments, discussing<br />problems, are few aspects of maintenance of communication. Clear and<br />unambiguous communication is an integral part of procedure compliance and safe<br />work practice. The following additional communication practices will be followed.<br />a. Repeat back is used to ensure accurate communication, especially when portable<br />radios, headsets, or telephones are being used.<br />b. Upon completion of a task, technicians shall report job completions to their<br />supervisors and seek additional assignments.<br />c. To confirm to the principle of solving problems at the lowest possible level,<br />potential grievance issues are to be discussed with the first line supervisor.<br />d. Plant approved terminology, equipment identification and abbreviations are to be<br />used at all times.<br />e. 2-way communication is required at times!<br />f. Listen<br />g. Understand<br />h. Then reply or repeat message.<br />Question and answers Electrical Maintenance Unit<br />- 412 -<br />• Importance of self checking peer verification<br />a. STAR Principle<br />S- Stop pause before performing a task,<br />T- Think Understand exactly what is to be done before taking any action.<br />A- Act Touch the component without actuating it. Then do it.<br />R- Review, verifies that the actual responses is the expected response.<br />b. Self-checking is a self-verification step or action before it is performed. This<br />behavior is developed through constant checking to ensure the intended action is<br />correctly and positively performed on the right equipment. Consistently applied this<br />will minimize error by forming a barrier against complacency and over confidence.<br />All are responsible for conducting self-checking prior to manipulating a component<br />or devices, or altering equipment configuration. For examples relays, positioning<br />switches, breaker or valves, lifting/landing wires, connecting test equipment,<br />removing or installing fuses.<br />c. Any deficiency found in the field like labels, nameplate missing/tampered<br />should be intimated to the supervisor.<br />d. Do it right the first time.<br />e. Peer verification is achieved through the use of inspection points, these include<br />dual verification, independent verification, supervisory verification and quality<br />verification. Peer verification leads to a broader concept of checking other.<br />f. Questioning attitude should develop for continuously learning.<br />Question and answers Electrical Maintenance Unit<br />- 413 -<br />The distinction between predictive and periodic maintenance is presented below.<br />a. Use predictive maintenance results to trend and monitor equipment performance<br />so that needed corrective or preventive maintenance can be performed before<br />equipment failure.<br />b. Predictive maintenance actions are determined by the data required to monitor<br />equipment condition.<br />Examples are as follows:<br />Vibration analysis (includes spectral analysis and bearing temperature<br />monitoring) and lubrication oil and grease analysis are used to monitor rotating<br />equipment.<br />Infrared surveys (thermography) are performed on heat producing equipment<br />such as motors, circuit breakers, batteries, load centers, bus ducts, transformers<br />and insulated areas to monitor for high resistance or insulation breakdown.<br />Oil analyses are performed on lubrication for rotating equipment to identify<br />degrading equipment and chemical breakdown of lubricants.<br />Motor operated valves are diagnostically tested and analysed. Tests determine<br />parameters such as run current, valve stem thrust and torque switch and limit<br />switch actuation points.<br />c. Periodic maintenance is time based action taken on equipment to prevent<br />breakdown and involves servicing such as lubrication, filter changes, cleaning,<br />testing, adjustments, calibration and inspection. Periodic maintenance can also be<br />initiated because of the results of predictive maintenance, vendor<br />recommendation, or experience. Examples are as follows:<br />a. Scheduled valve re-packing to avoid leakage based on previous experience.<br />b. Replacement of bearings or pump realignment as indicated from vibration<br />analysis and/or lubricating oil analysis<br />c. Major or minor overhauls based on experience or vendor<br />recommendations.<br />d. Maintenance on equipment belonging to a redundant safety system if so<br />allowed by the Technical Specifications<br />d. Preventive Maintenance Programme Effectiveness<br />Continually review the preventive maintenance programme for effectiveness, and<br />change if necessary based on changes in plant design, operating conditions,<br />regulatory commitments and as found conditions. In addition, unexpected<br />equipment failures should result in a critical self-assessment to determine why the<br />previous maintenance activities were insufficient to maintain equipment<br />reliability. The primary objectives of the programme are to reduce future<br />component failures, optimize preventive maintenance tasks and use of resources,<br />identify programme scope and satisfy regulatory and utility concerns. Emphasize<br />obtaining accurate feedback on preventive maintenance tasks. Enhancement,<br />provide additional guidance on methods to determine preventive maintenance<br />effectiveness.<br />Question and answers Electrical Maintenance Unit<br />- 414 -<br />Question and answers Electrical Maintenance Unit<br />- 415 -<br />• What are the advantages and disadvantages of planned maintenance?<br />Advantages of Planned Maintenance.<br />1. As the name reflects maintenance jobs planned properly ie. manpower, tools,<br />tackles, required for maintenance is well planned and readily available or<br />reserved for planned job<br />2. Procedures for doing the job in well known before doing the job and job can<br />be performed as per procedure/checklist.<br />3. It saves the time and unplanned outages of equipment.<br />4. Common facilities/tools/tackles/ in the section in the section can be made<br />available at the right time as job and requirements for that are already well<br />planned.<br />5. Overtime to employees can be limited.<br />6. Job can be done systematically, accurately as quality job can be expected.<br />Disadvantages of Unplanned maintenance.<br />1. Unplanned job won’t have any expectations when to start, when to stop.<br />2. Man power/tools tackles were available/not available at the right time is not<br />ensured.<br />3. Job may have to do in hurry which can lead to mistaken or job can be done<br />leisurely (no sufficient work front for the available manpower.) so wastage of<br />man machine tools etc.<br />4. In NPP we cannot accept unplanned jobs, as all works are safety<br />related/important.<br />• What is pre-job briefing and post job briefing?<br />Pre-job briefing: Unit no, DR/WP/, USI/system/load, Job description, Eqpt history,<br />Scope of Job, Any special tool or equipment required, Safety/Alarm, Procedures,<br />expectation for the job, tech specifications requirements, communication, FME<br />requirements, environmental concepts, any abnormal conditions.<br />Post job briefing: Details of work done, difficulties faced, deficiency found, parts<br />replaced, experience to be communicated, review modification, review procedure,<br />any suggestions, drawing updating, updating of history card, completion of<br />checklist, any testing/logic checks required, clearance for surrendering permit.<br />Question and answers Electrical Maintenance Unit<br />- 416 -<br />• What are the activities by which performance of the station will be judged?<br />Activities of station by which performance is judged by public<br />a. Capacity factor.<br />b. Availability factor.<br />c. Radiation release (gas and liquid effluents).<br />d. Thermal release.<br />e. Man-rem.<br />f. Development programs.<br />g. Public awareness.<br />h. Usefulness of the plant product to the public.<br />i. Employment and other facilities provided to the local public.<br />j. The Basic amenities provided to the employees.<br />k. The standard of living of the employees.<br />l. The profit earned by the Plant.<br />m. The quality and cleanliness in and around the Plant.bosehttp://www.blogger.com/profile/14555558365540483390noreply@blogger.com4